Make bullseye the base image

Prerequisite: https://gitlab.isc.org/isc-projects/images/-/merge_requests/132 & !5330 (merged)

Changes:

• makes bullseye the default image
• drops gcc:softhsm2.4 job as bullseye has softhsm 2.6 (as Fedora does already)

Issues:

• pylint - #2885 (closed) (fix: patch dnspython, see https://gitlab.isc.org/isc-projects/images/-/merge_requests/132/diffs?commit_id=e2a7edb59d95867e98490aebf79248916ab26a53)
• cppcheck - #2886 (closed) (fix: build cppcheck 2.2 for Bullseye or drop cppcheck altogether)
• tsan - #3010 (closed) (fix: custom libuv for Bullseye or move Clang from Bullseye to Fedora 35)
• respdiff - #2887 (closed) (fix: build BIND 9.11 with randomdev set, see !5367 (329b1681))
• gcov - gcovr reports 0 % coverage