Follow up tasks from #15565

• Handle generic methods
• Handle default kwargs for Python >= 3.13
• Unquote type variable forward reference annotations (see below)
• Handle TypeVarTuple generics with Unpack instead of *: class MultipleGenerics(Generic[S, T, Unpack[Ts], P]): ... ¹
• Allow multiple base classes for generic classes ([pyupgrade] Handle multiple base classes for PEP 695 generics (UP046) #15659)
• Use string replacements for UP040's autofix too, instead of using the ExprGenerator ([pyupgrade] Reuse replacement logic from UP046 and UP047 (UP040) #15840)
• Rename private type variables in generics (don't make inline generics like [_T]) ([pyupgrade] Rename private type parameters in PEP 695 generics (UP049) #15862)

Related: #4617, #12542