Migrate #solr IRC channel from Freenode to Libera

Due to a hostile takeover of the freenode IRC network, as detailed here, it has been proposed to move our IRC channel to the new Libera network (https://libera.chat/). Steps:

1. Officially register the #solr channel at libera.chat - see https://libera.chat/chanreg#community-registration
2. Move the slack integration from freenode to libera
3. Change our website to point to the new channel
4. Change topic in freenode #solr channel with link to the new location (can an IRC channel be made Read-only?)