ci: Upgrade CI images to Debian bullseye
Debian bullseye replaces buster as the latest stable version. We have upgraded Cloud Native GitLab to bullseye (gitlab-org/build/CNG!888 (merged)), so let's also upgrade CI to match.
Since upgrading system libraries will likely break C extensions, we need to tie the cache key to the Debian version to ensure gems get recompiled for this platform.
Edited by Stan Hu