Make 2021.06 available as container image and/or tarball

[bedroge]

When 2021.12 is available, we should not just delete 2021.06, but use it for testing a still to be developed script that can export that version to tarballs / container images.

The resulting tarballs can be stored in, for instance, a S3 bucket. The container images could go into the GHCR and/or another registry (AWS, Docker Hub)

@ocaisa came up with a nice suggestion to replace the directory of such a version by a variant symlink, which allows users to extract the tarball to some location and configure the variant symlink (in their local CVMFS config) to point to that location. This will allow them to keep using a version that's no longer available on CVMFS.