Rollup merge of #123680 - compiler-errors:gen-kw, r=Nadrieril

Deny gen keyword in `edition_2024_compat` lints

Splits the `keyword_idents` lint into two -- `keyword_idents_2018` and `keyword_idents_2024` -- since each corresponds to a future-compat warning in a different edition. Group these together into a new `keyword_idents` lint group, and add the latter to the `rust_2024_compatibility` so that `gen` is ready for the 2024 edition.

cc `@traviscross` `@ehuss`

Author: matthiaskrgr