Auto merge of #148216 - cuviper:stable-next, r=BoxyUwU,jackh726

[stable] Further revert `Deref for Cow` to its 1.90.0 state

It's not clear whether it breaks anything to *not* have this bound,
but the conservative choice is to match the state as of 1.90.0.

See also:
https://github.com/rust-lang/rust/issues/147964#issuecomment-3446420995
https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/Can.20this.20change.20break.20code.20in.201.2E91.3F/with/547528362