api: stabilize ManagedChannelBuilder.useTransportSecurity (#10244)

* api: stabilize ManagedChannelBuilder.useTransportSecurity and add a note in Java doc

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

@@ -193,14 +193,16 @@ public abstract class ManagedChannelBuilder<T extends ManagedChannelBuilder<T>>
   }
 
   /**
-   * Makes the client use TLS.
+   * Makes the client use TLS. Note: this is enabled by default.
+   *
+   * <p>It is recommended to use the {@link ChannelCredentials} API
+   * instead of this method.
    *
    * @return this
    * @throws IllegalStateException if ChannelCredentials were provided when constructing the builder
    * @throws UnsupportedOperationException if transport security is not supported.
    * @since 1.9.0
    */
-  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/3713")
   public T useTransportSecurity() {
     throw new UnsupportedOperationException();
   }