make namedTupleCanEqual stable

[bishabosha]

Fixes #26242

How much have you relied on LLM-based tools in this contribution?

Not at all

How was the solution tested?

Covered by existing tests (this is a refactoring)

[WojciechMazur]

Shouldn't this go through @preview stage first?

[bishabosha]

Shouldn't this go through @preview stage first?

Cc @Gedochao @tgodzik
@WojciechMazur someone should make this process crystal clear in a document because i am seeing conflicting messages e.g. #24675 (comment)

e.g. https://nightly.scala-lang.org/docs/contributing/procedures/contributing-to-stdlib.html says nothing about preview/experimental

Edit:
if i go back to 3.7.0 release notes it does seem to suggest that new API should be experimental and then preview @sjrd (https://scala-lang.org/news/3.7.0/#preview-features)

[odersky]

The preview scheme looks very heavy for library changes. Preview is primarily for tooling to catch up, but library changes should have no bearance on tooling. So I tend to think preview is not necessary for library changes.