[EventEngine] Bump expiration (grpc#38840)

Closes grpc#38840

COPYBARA_INTEGRATE_REVIEW=grpc#38840 from drfloob:bum b6334fe
PiperOrigin-RevId: 730959177

Author: drfloob

src/core/lib/experiments/experiments.yaml

@@ -148,7 +148,7 @@
   description:
     Prevent the PosixEventEngine from calling grpc_init & grpc_shutdown on
     creation and destruction.
-  expiry: 2025/03/02
+  expiry: 2025/06/01
   owner: [email protected]
   test_tags: ["core_end2end_test", "cpp_end2end_test"]
 - name: promise_based_http2_client_transport