CI test linux://python/ray/serve/tests:test_grpc is consistently_failing

[aslonnie]

CI test linux://python/ray/serve/tests:test_grpc is consistently_failing. Recent failures:
- https://buildkite.com/ray-project/postmerge/builds/15723#019c0c08-ef20-4cc4-9c55-077ee956caef
- https://buildkite.com/ray-project/postmerge/builds/15723#019c0b94-3e4a-49c7-a60f-51ab452b8373
- https://buildkite.com/ray-project/postmerge/builds/15723#019c0b94-3e4c-4453-badc-80939bb4fc03

DataCaseName-linux://python/ray/serve/tests:test_grpc-END
Managed by OSS Test Policy

[aslonnie]

This test is now considered as flaky because it has been failing on postmerge for too long. Flaky tests do not run on premerge.