proc-macro-error2 crate is unmaintained

[dmitryvk]

Hi, it seems that proc-macro-error2 is unmaintained (GnomedDev/proc-macro-error-2#17) and it's triggering cargo-audit. It does not seem to have any security impact, but it has a code that will stop compiling in future (GnomedDev/proc-macro-error-2#13), so it might be a good idea to migrate off of it.

[gamma0987]

The RUSTSEC advisory lists manyhow and proc-macro2-diagnostics as alternatives, but both require rewriting how errors are reported in your proc macros. proc-macro-error3 is a maintained fork and drop-in replacement - same API, no code changes beyond the dependency rename, so the switch is minimal and low-risk.

I maintain proc-macro-error3 and use it in my projects, mainly https://github.com/gungraun/gungraun. Would you be open to this switch? I'm happy to submit a PR if that would help.