Do not attempt to deploy the site upon release.

With a "site URL" configuration in the "distribution management" section
of the POM, the Maven release plugin defaults to automatically run the
"site-deploy" goal during "release:perform". This is not desired here,
so we force the plugin to only run "deploy".

pom.xml

@@ -141,6 +141,9 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-release-plugin</artifactId>
         <version>3.1.1</version>
+        <configuration>
+          <goals>deploy</goals>
+        </configuration>
       </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>