[9.20] chg: ci: move "stress" test generation script to QA repo

Move the util/generate-stress-test-configs.py script from the BIND 9
source repository to the BIND 9 QA repository.  This simplifies the
maintenance of that script by eliminating the need to backport every
change applied to it to multiple branches.

Backport of MR !10585

Merge branch 'backport-michal/move-stress-test-generation-script-to-qa-repo-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10612