Update IRC network name in the CONTRIBUTING file

KDE has moved from freenode to Libera Chat, I'm updating the CONTRIBUTING file to reflect this.

However, the #oxygen channel in freenode only had two people when I prepared this commit, so I think this isn't active at all, and I doubt it will be any more active with the move to Libera. Maybe we should link to the mailing list alone, or to #kde-devel or #kde-vdg.