updated default run_tests max version to py313 (#37945)

The default versions to run tests on master using `run_tests.py` are usually set to min and max supported Python versions. Looks like I missed updating the max version to Python 3.13 when adding support recently

Closes #37945

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37945 from sreenithi:add_missing_py313_test_config 708aa70f2b
PiperOrigin-RevId: 688702425

tools/run_tests/run_tests.py

@@ -867,7 +867,7 @@ class PythonLanguage(object):
                 # Default set tested on master. Test oldest and newest.
                 return (
                     python38_config,
-                    python312_config,
+                    python313_config,
                 )
         elif args.compiler == "python3.8":
             return (python38_config,)