Raise the MSRV?

[stevefan1999-personal]

I see that the rust-toolchain.toml is using a version that is 3 years ago and it is very old now. Code rot making it very hard to keep up with new package, some even require 1.80 now. I think we should raise the MSRV to match a more modern version.

[kkysen]

This isn't quite a MSRV, as it's the maximum as well. We have the toolchain pinned to a specific nightly because a bunch of the tools use rustc internals, which change often between versions, and upgrading is difficult. That said, the published crates, namely c2rust-transpile do not depend on rustc internals anymore and can compile on stable, and we're in the process of splitting that out into separate stable and unstable workspaces in #1227 and hopefully we can land that soon. We're also working on resurrecting c2rust-refactor, which was abandoned and on an even older nightly than the pinned one, so we're working on bringing that up to at least the pinned nightly.

[Rua]

If the version is upgraded, Rust 1.83 would be a good version to target, because it's the one that stabilised taking mutable references in static/const contexts. That would allow eliminating all the UB const-to-mut casting that c2rust currently needs to do.