reproducible Debian: Disable creation of stretch pbuilder tarballs and

update README to mention builds on bullseye instead of stretch.

It is my understanding that we are no longer testing stretch, so stop building the stretch tarballs, as this uses network bandwidth, cpu, and disk space that could be better spent on actual builds! :)

Maybe there are other places to remove stretch as well, though it would be nice to keep the historical data and charts and such.