let_chains stabilization followups

[est31]

PR #132833 has stabilized let_chains on edition 2024 (previously tracked at #53667), but that's not the whole story. As the stabilization report writes:

this stabilization PR only makes it possible to use let chains on 2024 without that feature gate, it doesn't mark that feature gate as stable/removed. I would propose to continue offering the let_chains feature (behind a feature gate) for a limited time (maybe 3 months after stabilization?) on older editions to allow nightly users to adopt edition 2024 at their own pace. After that, the feature gate shall be marked as stabilized, not removed, and replaced by an error on editions 2021 and below.

Currently there is a bunch of use by the components making up the compiler. Not all are subtrees, and we should try to minimize forced updates via subtrees.

• Make #![feature(let_chains)] bootstrap conditional in compiler/ #140202
• Remove some unused #![feature]s rust-clippy#14738
• Remove #![feature(let_chains)] from library and src/librustdoc #140966
• miri: Update to edition 2024 miri#4311
• Remove let_chains feature as it is stable miri#4316
• edition 2024 and remove #![feature(let_chains)] rustc_codegen_gcc#662
• Remove #![feature(let_chains)] from libcore #141415
• (Maybe?) wait a couple of weeks until some time has passed

Once this is all done: #143214 which:

• removes usage of the feature gate from tests, either by upgrading the edition, or by removing the variant or the entire test
• marks the feature gate as stabilized, remove gating, and introduce a <=2021 specific error when trying to use let_chains.

[est31]

Update: fwiw, I haven't removed it from tests yet because:

a) in the past month I was busy with life and work things, and
b) it was useful to compare different drop orders on different editions during the stabilization discussions of match if let guards, and compare it with the behaviour of match if let guards.

For b: maybe we can introduce a perma-unstable (and incomplete-linted for) feature to enable let chains on lower editions? We wouldn't suggest it either, as the feature would only be meant for writing tests... For a, stuff will change for the better in the coming month, but not sure by how much. Let's see.