red_knot_python_semantic: remove the "old" secondary message type

This finally completes the deletion of all old diagnostic types.

We do this by migrating the second (and last) use of secondary
diagnostic messages: to highlight the return type of a function
definition when its return value is inconsistent with the type.

Like the last diagnostic, we do actually change the message here a bit.
We don't need a sub-diagnostic here, and we can instead just add a
secondary annotation to highlight the return type.

crates/red_knot_python_semantic/resources/mdtest/function/return_type.md

@@ -310,7 +310,7 @@ def f(cond: bool) -> str:
     return "hello" if cond else NotImplemented
 
 def f(cond: bool) -> int:
-    # error: [invalid-return-type] "Object of type `Literal["hello"]` is not assignable to return type `int`"
+    # error: [invalid-return-type] "Return type does not match returned value: Expected `int`, found `Literal["hello"]`"
     return "hello" if cond else NotImplemented
 ```