proc-macro-error is unmaintained

[evie-calico]

As of September 5th of 2024, a rustsec "unmaintained" advisory has been issued for proc-macro-error.
https://rustsec.org/advisories/RUSTSEC-2024-0370

This isn't an urgent issue (it hasn't received an update in 4 years at this point), but I figured it would be useful to bring to your attention. It's also worth noting that this dependency of mlua is the only thing in my project that is currently still using syn v1.0.109