Rollup merge of #134076 - GrigorenkoPV:InvalidFilename, r=joboet

Stabilize `std::io::ErrorKind::InvalidFilename`

FCP complete: rust-lang/rust#130192 (comment)

Tracking issues: #86442 & #130192

This PR:
- Stabilizes `InvalidFilename` without changing its name to `InvalidFileName`.
- Fixes the doc comment (rust-lang/rust#130192 (comment))
- Does not separate "the name is too long" case into a separate `ErrorKind`. That case is currently documented to be covered by `InvalidFilename` in the doc. I'm not sure if it would be possible to change this later or if that would be an unacceptable breaking change.
- Does not map more raw OS errors to this `ErrorKind` (as suggested in rust-lang/rust#86442 (comment)). This can presumably be addressed later.

Author: Manishearth