mesa: bump rust req to 1.76

This is according to rusticl's rust update policy, as the current FireFox ESR requires 1.76.

I've also moved some of the rust handling into the global meson.build file and also moved the Rust update policy.

We might also want to consider bumping to 1.77 (for "C-string literals" and offset_of!) or 1.78 (for being able to assert in CI that some unsafe preconditions are met, see https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html#asserting-unsafe-preconditions), but for that I think we need to change the Rust update policy to reflect that bumping the version might also occur out of technical reasons as well.