Upgrade infra to Debian 11 (bullseye)
As per the Roadmap session done during Summit 2020.
Stretch LTS is supported from 6th July 2020 to June 30, 2022.
Note that upgrading directly from Debian N to N+2 is not supported, so one has to first upgrade any Stretch system to Buster, and then (possibly immediately) upgrade it to Bullseye.
jenkins: blocked by #17798