Update to syn-2

[maurer]

Motivation

Migration to syn-2 helps avoid multiple versions of syn going into the same project. The main tokio repo has already moved.

Solution

Aside from syn API updates, the two primary changes are:

1. self no longer parses as an Ident, so a compatibility shim to allow it to appear in skip was required.
2. MSRV is bumped to 1.56 for tracing-attributes due to the syn crate's MSRV.

[davidbarsky]

I think we'll get moved over to syn 2.0 once tracing-opentelemetry has been fully moved over to the new repo.

[hawkw]

See also the existing PR #2516 (which is blocked on getting CI un-broken, as David mentioned...)

[davidbarsky]

Oh, yeah—I think David got to the syn update first. I'll close this PR in favor of that one. Thanks for you help, though!