Improve Maven build and GitHub Maven workflow (#2079)

* Fix consecutive Maven builds failing without performing `clean`

By default moditect-maven-plugin refuses to overwrite the JAR file it
generated in a previous run.

* Make GitHub Maven build workflow detect Javadoc issues

.github/workflows/build.yml

@@ -15,4 +15,5 @@ jobs:
           java-version: '11'
           cache: 'maven'
       - name: Build with Maven
-        run: mvn --batch-mode --update-snapshots verify
+        # This also runs javadoc:javadoc to detect any issues with the Javadoc
+        run: mvn --batch-mode --update-snapshots verify javadoc:javadoc

gson/pom.xml

@@ -86,6 +86,8 @@
               <module>
                 <moduleInfoFile>${project.build.sourceDirectory}/module-info.java</moduleInfoFile>
               </module>
+              <!-- Overwrite the previously generated JAR file, if any -->
+              <overwriteExistingFiles>true</overwriteExistingFiles>
             </configuration>
           </execution>
         </executions>