api: Stabilize the SynchronixationContext class (#10130)

Author: temawi

api/src/main/java/io/grpc/SynchronizationContext.java

@@ -57,7 +57,6 @@
  * @since 1.17.0
  */
 @ThreadSafe
-@ExperimentalApi("https://github.com/grpc/grpc-java/issues/4984")
 public final class SynchronizationContext implements Executor {
   private final UncaughtExceptionHandler uncaughtExceptionHandler;