Target different downstreams based on the upstream source

With v2022 being rebased to Debian Bullseye we need to differentiate our target downstream branch when pulling updates from upstream.

In particular updates from Bullseye should currently go to the temporary bootstrap channel, while v2022dev1 is still based on Buster.

Later, updates from Buster should default to v2021 as it's the last Buster-based Apertis release, while updates from Bullseye should target the default branch starting from v2022dev2.

Task: T6873