Update Debian from buster to bullseye on CI

Why is this change being made?

Recently gitlab-org/gitlab-build-images!575 (merged) updated Debian version in the image used in this repo from buster to bullseye.

It seems safe as cache for both buster and bullseye to date, but cache keys should be updated as it is still confusing. While dropping OS version from the key name, it was avoided for its use in the future.

Follows up gitlab-org/gitlab-build-images!575 (merged)

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