fix: remove @BetaApi from ApiFutures and ApiService (#2454)

fixes #2098

Author: zhumin8

api-common-java/src/main/java/com/google/api/core/ApiFutures.java

@@ -161,7 +161,6 @@ public static <V, X extends Throwable> ApiFuture<V> catching(
    *     callback}
    * @see Futures#catchingAsync(ListenableFuture, Class, AsyncFunction, Executor)
    */
-  @BetaApi
   public static <V, X extends Throwable> ApiFuture<V> catchingAsync(
       ApiFuture<V> input,
       Class<X> exceptionType,

api-common-java/src/main/java/com/google/api/core/ApiService.java

@@ -165,7 +165,6 @@ public interface ApiService {
    * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
    * {@code A -> B}.
    */
-  @BetaApi
   enum State {
     /**
      * A service in this state has encountered a problem and may not be operational. It cannot be
@@ -198,7 +197,6 @@ enum State {
    *
    * <p>All methods are no-ops by default, implementors should override the ones they care about.
    */
-  @BetaApi
   abstract class Listener {
     /**
      * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The