[experiments] Update expiry of local_connector_secure experiment.

Author: matthewstevenson88

src/core/lib/experiments/experiments.yaml

@@ -157,7 +157,7 @@
   test_tags: []
 - name: local_connector_secure
   description: Local security connector uses TSI_SECURITY_NONE for LOCAL_TCP connections.
-  expiry: 2025/07/31
+  expiry: 2025/09/30
   owner: [email protected]
   test_tags: ["core_end2end_test"]
 - name: max_inflight_pings_strict_limit