do not expose flexible types

[hamzaremmal]

Do not infer public flexible type in the stdlib.
This is the only remaining exposed flexible type in the library:

[warn] -- Warning: /Users/hamzaremmal/Desktop/scala/scala3/library/src/scala/util/control/NoStackTrace.scala:27:42 
[warn] 27 |    if (NoStackTrace.noSuppression) super.fillInStackTrace()
[warn]    |                                          ^
[warn]    |method scala$util$control$NoStackTrace$$super$fillInStackTrace exposes a flexible type in its inferred result type (Throwable)?. Consider annotating the type explicitly

[noti0na1]

The remaining flexible types are either inside anonymous classes or at private/protected definitions, so I didn't fix them. I was planning to update the flexible type checker later. Do we have to fix them now?

[hamzaremmal]

The main reason I opened this PR is to reduce the number of warnings. I didn't go in depth into the logic behind the warning and if they should have been a warning in the first place.

[noti0na1]

I think it is fine to merge this. We will update the checker later so we don't report warnings on definitions that are not really public, for example, members of anonymous or private classes.

[sjrd]

The remaining flexible types are either inside anonymous classes or at private/protected definitions

protected definitions are part of the public API. We shouldn't have flexible types in them either.