error codes section is outdated

[cyrgani]

https://rustc-dev-guide.rust-lang.org/diagnostics/error-codes.html mentions things that no longer exist, such as register_diagnostics! (removed in rust-lang/rust#119972)

[tshepang]

@cyrgani do you have capacity to submit a fix (for at least some of the stale things)

[cyrgani]

I can do that once rust-lang/rust#146584 is merged, since that PR will change the macro syntax again.