Sysadmin follow-ups to the migration of the Tails chats away from XMPP

Hi!

• Please set the topic of the tails-dev XMPP room to this text: This room is obsolete. See you on Matrix: https://matrix.to/#/#tails-dev:matrix.org
• Please set the topic of the tails XMPP room to this text: This room is obsolete. See you on IRC: https://tails.net/support/chat/
• You can now stop running the XMPP bot called tailsbot in the tails-dev XMPP room

Context: https://gitlab.tails.boum.org/tails/tails/-/issues/20995, https://gitlab.tails.boum.org/tails/tails/-/issues/20996

Thanks in advance!

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Some more steps:

• delete documentation
• delete leftover rooms (assembly, bar, board, sysadmins)
• delete the tailsbot XMPP user