Move official IRC presence to Libera

[meejah]

There have been some changes at Freenode and most ex-freenode-staff have moved to Libera. We should follow suit and move our official IRC channel to #tahoe-lafs on Libera.

People have been pro-active about granting ops etc into channels that exist on both. Other considerations:

• de-commission the Freenode presence? How?
  • keep freenode channel "alive"
  • change topic
  • nobody can post; must go to libera
• update docs to point at Libera + #tahoe-lafs instead of Freenode.
• Slack bridge should point to Libera (possibly "also point to Libera" if that's easy)

We agreed that it's also important to show support for the ex-freenode volunteers by moving to Libera. Other communications infrastructure (e.g. Zulip was mentioned, OFTC was mentioned) can be decided upon or changed later.