Migrate from legacy com.google.gwt to org.gwtproject.

org.gwtproject already has a 2.11.0 release available, which I'm not upgrading to in this CL. I'll leave that to Dependabot (and future semi-automated Guava version bumps).

See google/auto#1342 (comment)

RELNOTES=n/a
PiperOrigin-RevId: 647700630

guava-gwt/pom.xml

@@ -345,6 +345,7 @@
           </execution>
         </executions>
       </plugin>
+      <!-- TODO(cpovirk): We really should try tbroyer's plugin instead of this legacy one. -->
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>gwt-maven-plugin</artifactId>