Upgrading buildbot and switching to Debian 11

[Petr Štetiar]

Hi,

FYI, just started moving master buildbot server to a new hardware and will
move snapshot builds there first to iron out any remaining issues.

While at it I'm updating it to the latest buildbot 3.8.0 release and switching
base container images from Debian 10 to Debian 11[1]:

 "Debian 10 LTS support ends on 6/2024, so it makes no sense to use it as
  a base for 23.05 release, so lets switch to Debian 11 which should've
  LTS support till 6/2026."

I'll try to keep you updated during transition.

1. https://github.com/openwrt/buildbot/pull/3/commits/f2c27d0084c9450e86e6534cc9ea2fcc92aa0828


Cheers,

Petr