Consider adding Debian bullseye release package

Description

The gitlab-runner APT package in the Debian buster distribution release repository seems to work perfectly on Debian bullseye, which is nice.

That said, the installation script will - by default - write the current OS release name to the APT sources list file, so it places bullseye in there for users on that release.

It's not a hard workaround for users to edit that file, but perhaps it would be nice to package a bullseye version (at least for recent builds; perhaps not for long-term supported builds).

Proposal

Add debian/bullseye to the list of packagecloud Deb release platforms.

I'll offer a merge request to do this, although I fully expect to find out that the situation is more complicated than a one-liner can address :)

Links to related issues and merge requests / references

Partially related to !1536 (merged) - I found some of the commentary there useful for context.