Skip broken tag links on offline gc

Extracted from: https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/30

When running garbage collection, if there are corrupted/broken tags on the storage backend the garbage collection fails with failed to garbage collect: marking blobs: 1 error occurred: * retrieving tags invalid checksum digest format.

This is triggered here.

In this issue we would like to simply skip these broken tags by default. We already do this in the importer. When the time comes for migration on self-managed, those tags will be ignored anyway. And now, if they are broken, it is already as if they did not exist.