Warn on updates to compiler targets

[jhwgh1968]

Closes #310.

This was simpler than I thought, but also has some reductions:

• I did not know that multi-line messages were discouraged by the structure of the code. I decided to work within that, rather than refactor it just for this new message.
• Given the pattern match I put in, I decided not to tag release notes to avoid false positives. I am not sure if, for example, some LLVM changes would trigger this warning as well.

This is ready for review, but am putting it in draft because the RFC I'm linking to is not yet merged. Once that happens (it seems to be a matter of when at this point), I will update it with the proper link on The Forge.

[jhwgh1968]

Actually, it's not quite on the website yet. Based on rust-lang/rust#84583, I think the link will be:
https://doc.rust-lang.org/rustc/target_tier_policy.html

(Why can't see the button to make it a draft again? Oh well.)

[joshtriplett]

It's on the site now: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html

Nightly will always have the latest version of the policy, so I'd suggest linking there.

[jhwgh1968]

Apparently first-time contributors need a maintainer to run CI. Could I get a run please?

[jhwgh1968]

Randomly pinging @oli-obk who committed recently, could I get a review and a CI run?

[jhwgh1968]

Randomly pinging @pietroalbini who committed recently. Could I get a review and a CI run? This has been sitting for weeks.

[pietroalbini]

Hmm, the button to approve the CI run is not appearing anymore... Let me try closing and reopening the PR.

[pietroalbini]

Thanks!