GitLab 19.0 with GitLab container registry (GLCR) 4.40 starts migrating
to a real database as metadata backend. We should follow that and offer
our users a migration way to this backend. CNCF Distribution (previously
known as Docker Distribution) doesn't support this kind of metadata
storage and it also seems off to implement this in the
`services.dockerRegistry` NixOS module as the GLCR
moved quite far away from CNCF Distribution.
This change additionally changes the default user GLCR runs in to make it
less confusing for users, especially as the user is also now used to
connect to PostgreSQL.
This change also modifies the module for GitLab 19.0 / GLCR 4.40 support.
