upgrade dependency proc-macro2

[jtmoon79]

tl;dr upgrading proc-macro2 version would avoid a known proc-macro2 bug error[E0635]: unknown feature proc_macro_span_shrink

My project is using phf v0.11.2 depends on proc-macro2 v1.0.52.

$ cargo tree
...
├── phf v0.11.2
│   ├── phf_macros v0.11.2 (proc-macro)
│   │   ├── phf_generator v0.11.1
│   │   │   ├── phf_shared v0.11.2
│   │   │   │   └── siphasher v0.3.10
│   │   │   └── rand v0.8.5
│   │   │       └── rand_core v0.6.4
│   │   ├── phf_shared v0.11.2 (*)
│   │   ├── proc-macro2 v1.0.52 (*)
│   │   ├── quote v1.0.26 (*)
│   │   └── syn v2.0.11 (*)
│   └── phf_shared v0.11.2
│       └── siphasher v0.3.10

proc-macro2 Issue dtolnay/proc-macro2#356 causes failures of some tools like udeps with error

$ cargo +nightly udeps
error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/ulug/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.52/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

According to the linked proc-macro2 Issue

you need to upgrade proc-macro2 to 1.0.60+, downgrade proc-macro2 to 1.0.43 or before, downgrade the nightly toolchain to nightly-2023-06-28 or before, or use a non-nightly toolchain.

This error occurred on rust 1.70.0 and 1.72.0.

Can rust-phf upgrade it's proc-macro2 dependency version?

[JohnTitor]

I'm happy to see a PR :)