Fix links to mailing lists

[kwin]

As all the mailing lists currently outlined in the pom.xml are dead and the only relevant one is now https://groups.google.com/g/mojohaus-dev according to https://github.com/codehaus-plexus/codehaus-plexus.github.io/blob/53ac43e79ecd42f1c7410b640f0079b53021dc66/src/site/site.xml#L65 this should be reflected in the pom.xml as well.