upgrades: Starts dist-upgrade when system is Debian testing

In a testing container, I noticed that start-dist-upgrade would attempt to start a dist upgrade, although there is nothing to be upgraded to.

# lsb_release --release --codename --short
11
bullseye

start-dist-upgrade should report "already testing" and not start the dist upgrade.

# ./upgrades start-dist-upgrade
{"dist_upgrade_started": true, "reason": "started-dist-upgrade"}