Unbalanced prioritization in the images buildbot? (main branch deprioritized too much)

[Hannu Nyman]

Looks like the release branches might have a too strong priority in the 
combined image buildbot, so that release branches get always built before the 
development main/master.

Recently there has been a steady flow of mostly small/unessential 
fixes/improvements for 22.03 and 23.05, so buildbot has been focused on 
building 22.03-SNAPSHOT and 23.05-SNAPSHOT (and the ongoing 23.05.1 build), 
and the main branch builds do lag really badly.

There was a bug in netifd WLAN device / hotplug handling that Felix fixed 
already 3 days ago, but we are still offering faulty downloads for half of 
the targets in main, as buildbot is prioritising  22.03 and 23.05 due to new 
commits in them. Oldest (and faulty) main build downloads are currently from 
Wed 8th = 5 days ago. That affects the imagebuilder/auc/attendedsysupgrade 
users, too.

The combined queue for images looks nice in principle, but can apparently 
lead into discrimination of main/master, if there are steadily new commits to 
the dormant stable branches.

There should maybe be some sanity check that each target gets built at least 
after X days since the last build, even if there are newer commits in the 
prioritized release branches. The "time since last build" should have a 
growing weight.