Refactor: replace make_simplified_union with UnionType.make_union in binder.py (#8624)

This commit replaces all calls to make_simplified_union with UnionType.make_union in this file, as part of the codebase modernization effort in issue #8624.

Author: lsrafael13

mypy/binder.py

@@ -277,7 +277,7 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                     # interfere with our (hacky) TypeGuard support.
                     type = possible_types[0]
                 else:
-                    type = make_simplified_union(possible_types)
+                    type = UnionType.make_union(possible_types)
                     # Legacy guard for corner case when the original type is TypeVarType.
                     if isinstance(declaration_type, TypeVarType) and not is_subtype(
                         type, declaration_type