Skip maven-gpg-plugin in the gwtserializer module.

I'm not sure why this has started mattering. Perhaps because of an update to the plugin. But without this change we get this when trying to deploy AutoValue:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-gpg-plugin:3.1.0:sign (sign-artifacts) on project gwtserializer: The project artifact has not been assembled yet. Please do not invoke this goal before the lifecycle phase "package". -> [Help 1]
```

RELNOTES=n/a
PiperOrigin-RevId: 544057275

value/src/it/gwtserializer/pom.xml

@@ -150,6 +150,13 @@
           <skip>true</skip>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-gpg-plugin</artifactId>
+        <configuration>
+          <skip>true</skip>
+        </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>