nixos/doc: Mention container registry deprecation in gitlab

Support for external container registries has ended in GitLab >=16 and
module users are therefore advised to switch to gitlab's fork instead.

nixos/doc/manual/release-notes/rl-2305.section.md

···
 
 - `gitlab` has been upgraded from major version 15 to major version 16 and requires at least PostgreSQL 13.6. Check the [upgrade guide](#module-services-postgres-upgrading) in the NixOS manual on how to upgrade your PostgreSQL installation.
 
+- `gitlab` 16 deprecates the use of external container registries, in our case `pkgs.docker-distribution`. Module users who have [`services.gitlab.registry.enable`](#opt-services.gitlab.registry.enable) set to `true` are advised to back up their state and switch to gitlab's fork by setting [`services.gitlab.registry.package`](#opt-services.gitlab.registry.package) to `pkgs.gitlab-container-registry`.
+
 - `fail2ban` has been updated to 1.0.2, which has a few breaking changes compared to 0.11.2 ([changelog for 1.0.1](https://github.com/fail2ban/fail2ban/blob/1.0.1/ChangeLog), [changelog for 1.0.2](https://github.com/fail2ban/fail2ban/blob/1.0.2/ChangeLog))
 
 - `albert` has been updated from 0.17.6 to 0.20.13, and 0.18.0 changed the config format and many plugins ([changelog for 0.18.0](https://github.com/albertlauncher/albert/blob/v0.18.0/CHANGELOG.md))