Fix error code of note about unbound type variable (#21668)

The note used the default `misc` error code, which was different from
the error message.

Author: JukkaL

mypy/checker.py

@@ -1825,6 +1825,7 @@ def check_unbound_return_typevar(self, typ: CallableType) -> None:
                         "Consider using the upper bound "
                         f"{format_type(typ.ret_type.upper_bound, self.options)} instead",
                         context=typ.ret_type,
+                        code=TYPE_VAR,
                     )
 
     def check_default_params(self, item: FuncItem, body_is_trivial: bool | None = None) -> None:

test-data/unit/check-typevar-unbound.test

@@ -13,6 +13,9 @@ def g() -> U: # E: A function returning TypeVar should receive at least one argu
               # N: Consider using the upper bound "int" instead
     ...
 
+def g2() -> U:  # type: ignore[type-var]
+    ...
+
 V = TypeVar('V', int, str)
 
 def h() -> V:  # E: A function returning TypeVar should receive at least one argument containing the same TypeVar