Make `configure_workers_and_start` script used in Complement tests compatible...

Make `configure_workers_and_start` script used in Complement tests compatible with older versions of Python. (#15275)

changelog.d/15275.misc

+Make `configure_workers_and_start` script used in Complement tests compatible with older versions of Python.
\ No newline at end of file

docker/configure_workers_and_start.py

@@ -422,7 +422,7 @@ def add_worker_roles_to_shared_config(
 
 
 def merge_worker_template_configs(
-    existing_dict: Dict[str, Any] | None,
+    existing_dict: Optional[Dict[str, Any]],
     to_be_merged_dict: Dict[str, Any],
 ) -> Dict[str, Any]:
     """When given an existing dict of worker template configuration consisting with both