Build Debian bullseye version for Docker image
What does this MR do and why?
This would fix build failures in https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/6153546871.
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Stan Hu