Drop generic_tests image support for old releases

Currently we build the generic_tests image for all Debian releases, including old ones. This doesn't provide any real benefit, since the generic tests (rc-bugs, missing-breaks and wrap-and-sort) do not depend on release-specific differences, they only need a build environment, and using unstable is sufficient for that.

Building and maintaining images for older releases adds extra overhead and increases pipeline time. For that reason, I think we should drop support for generic_tests images on old releases and only keep the unstable(sid) one.