api: Stabilize io.grpc.CallCredentials

[temawi]

Removes the @ExperimentalApi annotation from the class and provides an empty implementation for the thisUsesUnstableApi() method.

[temawi]

cc: @ejona86

[larry-safran]

You should put a note in issues/1914 to remove the method and schedule closing the issue for 1.57

[temawi]

You should put a note in issues/1914 to remove the method and schedule closing the issue for 1.57

I made a note on that issue that the class is stabilized and that the method is deprecated. The issue is still open, but I did not specifically schedule it for 1.57 yet. How do we determine that that is sufficient time for people to stop using the method?

[ejona86]

Let's preserve the @ExperimentalApi annotation on the thisUsesUnstableApi() method, to make it clear we can remove the method.