nixos/gitlab: Add support for gitlab-container-registry

Support for external container registries (namely
`pkgs.docker-distribution`) has ended in GitLab 16.0 [1] and is
scheduled for removal in a future release. [2]

This commit adds a new registry package option, defaulting to
`pkgs.docker-distribution` and `pkgs.gitlab-container-registry` for
system state versions >= 23.11.

While it is still possible to use the docker container registry, module
users should switch to gitlab's container registry fork soon. A warning
message will be added in a future commit to advise affected users to
back up their state and then switch to the new container registry
package.

[1]: https://gitlab.com/gitlab-org/gitlab/-/issues/376217
[2]: https://gitlab.com/gitlab-org/gitlab/-/issues/403322

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