Rollup merge of #137653 - tgross35:deprecate-concat_idents, r=workingjubilee

Deprecate the unstable `concat_idents!`

`concat_idents` has been around unstably for a long time, but there is now a better (but still unstable) way to join identifiers using `${concat(...)}` syntax with `macro_metavar_expr_concat`. This resolves a lot of the problems with `concat_idents` and is on a better track toward stabilization, so there is no need to keep both versions around. `concat_idents!` still has a lot of use in the ecosystem so deprecate it before removing, as discussed in [1].

Link: rust-lang/rust#124225

[1]: https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/Removing.20.60concat_idents.60

Author: matthiaskrgr