Migrate riverside to bullseye

That's the sentry node.

Plan, as root in riverside:

task=#3761
puppet agent --disable "$task: dist-upgrade to bullseye"
cd /etc
sed -i -e 's/buster/bullseye/' /etc/apt/sources.list.d/*
sed -i -e 's,bullseye/updates,bullseye-security,' /etc/apt/sources.list.d/debian-security.list
git status
git diff
grep bullseye-security /etc/apt/sources.list.d/debian-security.list
git add .
git commit -m "$task: Migrate sources.list to bullseye"
apt update
apt upgrade -y
apt dist-upgrade -y
reboot
puppet agent --enable && puppet agent --test

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Migrated from T3761 (view on Phabricator)