[10213] Document the usage of Libera.chat.

[adiroiban]

Scope and purpose

We no longer use freenode but libera.chat

I have only updated the freenode references from RST files and not from the example .py files.

Not sure if we should do a freenode purge. I can do that if you think that is a good idea.

Contributor Checklist:

• The associated ticket in Trac is here: https://twistedmatrix.com/trac/ticket/10213
• I ran tox -e lint to format my patch to meet the Twisted Coding Standard
• I have created a newsfragment in src/twisted/newsfragments/ (see: News files)
• The title of the PR starts with the associated Trac ticket number (without the # character).
• [NA] I have updated the automated tests and checked that all checks for the PR are green.
• I have submitted the associated Trac ticket for review by adding the word review to the keywords field in Trac, and putting a link to this PR in the comment; it shows up in https://twisted.reviews/ now.
• The merge commit will use the below format
  The first line is automatically generated by GitHub based on PR ID and branch name.
  The other lines generated by GitHub should be replaced.

Merge pull request #123 from twisted/4356-branch-name-with-trac-id

Author: adiroiban
Reviewer: 
Fixes: ticket:10213

Remove freenode from the documentation/README and use Libera.chat.

[hynek]

according to https://twistedmatrix.com/pipermail/twisted-python/2021-July/065630.html there doesn't seem to be a #twisted.web anymore?

[adiroiban]

Thanks for the review.

I have joined the #twisted.web channel just now ...and I am not alone on the channel.
I think that #twisted.web is alive.

I think that Tom only gave a few examples.

But I can see that #twisted-infra irc was empty... and on the wiki I have updated it with twisted-dev