Remove usage of deprecated logger.warn method from codebase (#6271)

Replace every instance of `logger.warn` with `logger.warning` as the former is deprecated.

synapse/rest/client/v1/login.py

@@ -221,7 +221,7 @@ class LoginRestServlet(RestServlet):
                 medium, address
             )
             if not user_id:
-                logger.warn(
+                logger.warning(
                     "unknown 3pid identifier medium %s, address %r", medium, address
                 )
                 raise LoginError(403, "", errcode=Codes.FORBIDDEN)