Use actual PHP 8.1 Enums in the `EnumTypeResolver`, once Rector fully supports downgrading it

Currently Rector does not fully support downgrading PHP 8.1 Enums (see https://github.com/rectorphp/rector/issues/7013#issuecomment-1228147159).

When full support is provided, continue working on branch [Use-Enum-in-EnumTypeResolver](https://github.com/leoloso/PoP/tree/Use-Enum-in-EnumTypeResolver).