[experiments] extend work_serializer_dispatch expiry (#37127)

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Closes #37127

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37127 from yousukseung:extend-work-serializer-dispatch b63295336a
PiperOrigin-RevId: 648451105

src/core/lib/experiments/experiments.yaml

@@ -205,6 +205,6 @@
     Have the work serializer dispatch work to event engine for every callback,
     instead of running things inline in the first thread that successfully
     enqueues work.
-  expiry: 2024/06/30
+  expiry: 2024/09/30
   owner: ysseung@google.com
   test_tags: ["core_end2end_test", "cpp_end2end_test", "xds_end2end_test", "lb_unit_test"]