Identify and warn about non-legit uses of Wildcard Types #23732
Description
Wildcards, by virtue of being part of Java, are necessary in Scala for interoperability. But wildcards are not well understood at all, and oftentimes, outside of some narrowly defined uses, they are not necessary or being abused.
Towards Scala 3.10, we should identify non-legit uses of wildcards in a best-effort manner and emit warnings.