Deprecate `-Yno-kind-polymorphism` and update the relevant docs

[OndrejSpanel]

When you read https://docs.scala-lang.org/scala3/reference/other-new-features/kind-polymorphism.html, at the end there is:

Note: This feature is considered experimental but stable

What does this mean? Can a feature be experimental and stable at the same time?

Expectation

AnyKind is either experimental. or stable, not both at the same time.

Requirements

• Update the docs on kind polymorphism
• Deprecate the -Yno-kind-polymorphism flag
  • We should remove it after a minor version bump or two

[Gedochao]

I am confused myself, I think it's likely to just be awkward phrasing...
@odersky do you recall what was meant here?
I agree we should improve the doc (and potentially reconsider the feature in the current context)

[sjrd]

AnyKind is fully stable. The Scala 3 type system makes no sense without AnyKind.

I think this is obsolete phrasing from a time where we kind of took in AnyKind before 3.0.0, not fully knowing whether it was all that important.

[Gedochao]

@sjrd is the -Yno-kind-polymorphism flag still relevant? If it's considered stable, should we promote it out of -Y?

[sjrd]

No we should remove that flag instead. Getting rid of kind polymorphism makes no sense anymore.

[Gedochao]

Understood.
I updated the issue requirements.