Switch base OS to Debian bullseye amd64

* Also drop python-yaml (and python)… package doesn't exist anymore and
  we probably don't need system python2?
* gawk 5 warns when escaping `~`… and it wasn't needed on older gawk
  versions

Depends-On: 15547
Change-Id: Ib8a25d112ba1bfd75b8a5d005f2cfd93323e9e9f