Migrate hedgedoc stack to bullseye

This means migrating the following nodes:

• rp1.internal.admin.swh.network: the reverse proxy in charge of publicly exposing hedgedoc
• bardo.internaladmin.swh.network: the actual node serving hedgedoc

plan:

$ puppet agent --disable '#3749'
$ sed -i -e 's/buster/bullseye/' /etc/apt/sources.list.d/*
$ sed -i -e 's,bullseye/updates,bullseye-security,' /etc/apt/sources.list.d/debian-security.list
$ apt update
$ apt upgrade -y
$ apt dist-upgrade -y
$ reboot
$ puppet agent --enable && puppet agent --test
# check everything still runs fine

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Migrated from T3749 (view on Phabricator)