tests: Remove public modifier in protobuf 3 tests.

Author: blakeli0

java-showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITProtobuf3Compatibility.java

@@ -15,7 +15,7 @@
  * Tensorflow depends on protobuf 3.x gen code and runtime, we test it in showcase module to prove
  * that it works with protobuf 4.33+ gen code and runtime that comes with client libraries.
  */
-public class ITProtobuf3Compatibility {
+class ITProtobuf3Compatibility {
 
   @Test
   void testTensorflow_helloWorldExample() {