Rollup merge of #141072 - Rynibami:stabilize-const-result-flatten, r=jhpratt

Stabilize feature `result_flattening`

Stabilizes the `Result::flatten` method

## Implementations

- [x] Implementation `Result::flatten`: rust-lang/rust#70140
- [x] Implementation `const` `Result::flatten`: rust-lang/rust#130692
- [x] Update stabilization attribute macros (this PR)

## Stabilization process

- [x] Created this PR [suggested](rust-lang/rust#70142 (comment)) by ``@RalfJung``
- [x] FCP (haven't found any, is it applicable here?)
- [ ] Close issue rust-lang/rust#70142

Author: jhpratt