chore: bump MSRV to 1.56 (#2546)

As part of upgrading syn to 2.0 (e.g.,
#2516), we need to bump the MSRV
to 1.56. As part of this PR, I've:
- Updated the text descriptions of what would be an in-policy MSRV bump
  to use more recent versions of rustc. The _niceness_ of said version
  numbers are purely coincidental.
- I've removed some of the exceptions made in CI.yml in order to support
  some crates with a higher MSRV.

tracing-mock/README.md

@@ -0,0 +1,176 @@
+![Tracing — Structured, application-level diagnostics][splash]
+
+[splash]: https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/splash.svg
+
+# tracing-mock
+
+Utilities for testing [`tracing`][tracing] and crates that uses it.
+
+[![Documentation (master)][docs-master-badge]][docs-master-url]
+[![MIT licensed][mit-badge]][mit-url]
+[![Build Status][actions-badge]][actions-url]
+[![Discord chat][discord-badge]][discord-url]
+
+[Documentation][docs-master-url] | [Chat][discord-url]
+
+[docs-master-badge]: https://img.shields.io/badge/docs-master-blue
+[docs-master-url]: https://tracing-rs.netlify.com/tracing_mock
+[mit-badge]: https://img.shields.io/badge/license-MIT-blue.svg
+[mit-url]: https://github.com/tokio-rs/tracing/blob/master/tracing-mock/LICENSE
+[actions-badge]: https://github.com/tokio-rs/tracing/workflows/CI/badge.svg
+[actions-url]:https://github.com/tokio-rs/tracing/actions?query=workflow%3ACI
+[discord-badge]: https://img.shields.io/discord/500028886025895936?logo=discord&label=discord&logoColor=white
+[discord-url]: https://discord.gg/EeF3cQw
+
+## Overview
+
+[`tracing`] is a framework for instrumenting Rust programs to collect
+structured, event-based diagnostic information. `tracing-mock` provides
+tools for making assertions about what `tracing` diagnostics are emitted
+by code under test.
+
+*Compiler support: [requires `rustc` 1.56+][msrv]*
+
+[msrv]: #supported-rust-versions
+
+## Usage
+
+`tracing-mock` crate provides a mock
+[`Collector`](https://tracing-rs.netlify.app/tracing/#collectors)
+that allows asserting on the order and contents of
+[spans](https://tracing-rs.netlify.app/tracing/#spans) and
+[events](https://tracing-rs.netlify.app/tracing/#events).
+
+As `tracing-mock` isn't available on [crates.io](https://crates.io/)
+yet, you must import it via git. When using `tracing-mock` with the
+`tracing` `0.1` ecosystem, it is important that you also override the
+source of any `tracing` crates that are transient dependencies. For
+example, the `Cargo.toml` for your test crate could contain:
+
+```toml
+[dependencies]
+lib-under-test = "1.0" # depends on `tracing`
+
+[dev-dependencies]
+tracing-mock = { git = "https://github.com/tokio-rs/tracing", branch = "v0.1.x", version = "0.1" }
+tracing = { git = "https://github.com/tokio-rs/tracing", branch = "v0.1.x", version = "0.1" }
+
+[patch.crates-io]
+tracing = { git = "https://github.com/tokio-rs/tracing", branch = "v0.1.x" }
+tracing-core = { git = "https://github.com/tokio-rs/tracing", branch = "v0.1.x" }
+```
+
+## Examples
+
+The following examples are for the `master` branch. For examples that
+will work with `tracing` from [crates.io], please check the
+[v0.1.x](https://github.com/tokio-rs/tracing/tree/v0.1.x/tracing-mock)
+branch.
+
+Below is an example that checks that an event contains a message:
+
+```rust
+use tracing::collect::with_default;
+use tracing_mock::{collector, expect, field};
+
+fn yak_shaving() {
+    tracing::info!("preparing to shave yaks");
+}
+
+let (collector, handle) = collector::mock()
+    .event(expect::event().with_fields(field::msg("preparing to shave yaks")))
+    .only()
+    .run_with_handle();
+
+with_default(collector, || {
+    yak_shaving();
+});
+
+handle.assert_finished();
+
+```
+
+Below is a slightly more complex example. `tracing-mock` asserts that, in order:
+- a span is created with a single field/value pair
+- the span is entered
+- an event is created with the field `number_of_yaks`, a corresponding
+  value of 3, and the message "preparing to shave yaks", and nothing else
+- an event is created with the field `all_yaks_shaved`, a corresponding value
+  of `true`, and the message "yak shaving completed"
+- the span is exited
+- no further traces are received
+
+```rust
+use tracing::collect::with_default;
+use tracing_mock::{collector, expect, field};
+
+#[tracing::instrument]
+fn yak_shaving(number_of_yaks: u32) {
+    tracing::info!(number_of_yaks, "preparing to shave yaks");
+
+    let number_shaved = number_of_yaks; // shave_all
+    tracing::info!(
+        all_yaks_shaved = number_shaved == number_of_yaks,
+        "yak shaving completed."
+    );
+}
+
+let yak_count: u32 = 3;
+let span = expect::span().named("yak_shaving");
+
+let (collector, handle) = collector::mock()
+    .new_span(
+        span.clone()
+            .with_field(expect::field("number_of_yaks").with_value(&yak_count).only()),
+    )
+    .enter(span.clone())
+    .event(
+        expect::event().with_fields(
+            expect::field("number_of_yaks")
+                .with_value(&yak_count)
+                .and(field::msg("preparing to shave yaks"))
+                .only(),
+        ),
+    )
+    .event(
+        expect::event().with_fields(
+            expect::field("all_yaks_shaved")
+                .with_value(&true)
+                .and(field::msg("yak shaving completed."))
+                .only(),
+        ),
+    )
+    .exit(span.clone())
+    .only()
+    .run_with_handle();
+
+with_default(collector, || {
+    yak_shaving(yak_count);
+});
+
+handle.assert_finished();
+```
+
+## Supported Rust Versions
+
+Tracing is built against the latest stable release. The minimum supported
+version is 1.56. The current Tracing version is not guaranteed to build on Rust
+versions earlier than the minimum supported version.
+
+Tracing follows the same compiler support policies as the rest of the Tokio
+project. The current stable Rust compiler and the three most recent minor
+versions before it will always be supported. For example, if the current stable
+compiler version is 1.69, the minimum supported version will not be increased
+past 1.66, three minor versions prior. Increasing the minimum supported compiler
+version is not considered a semver breaking change as long as doing so complies
+with this policy.
+
+## License
+
+This project is licensed under the [MIT license][mit-url].
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Tracing by you, shall be licensed as MIT, without any additional
+terms or conditions.