Deprecate old reflect isErased API

[nicolasstucki]

Based on the changes in #16507 we need to update the API. We must deprecate:

• MethodTypeMethods.isErased
• TermParamClauseMethods.isErased
• TypeReprMethods.isErasedFunctionType
• defn.FunctionClass

We can only deprecate these methods in a minor release.

Originally posted by @nicolasstucki in #16507 (comment)

[nicolasstucki]

FunctionClass needs a replacement that does not take default arguments.