gitlab: loose coupling to postgresql and redis

[osnyx]

Description of changes

This reduces the strong degree of coupling between Gitlab and its supporting 3rd party services redis and postgresql.

Background:
When Gitlab and its database postgresql are running on the same system, its systemd services are strongly coupled via binds-to relations to ensure start/stop/restart of the individual services propagates to the other services.
We already had some slight issues with this in the past and fixed the reliability of coupled restarts in NixOS#245240. But since then, we still have seen some additional hard-to-reproduce race conditions that show the approach of strongly-coupled service restarts is brittle here. In one case when postgresql needed to restart, Gitlab was shut down while trying to start and it stayed inactive.
As a consequence, I propose to drop the approach of strong coupling between these services altogether and let the services postgresql or redis restart without forcing a restart of gitlab.service. This just causes the running service to temporarily loose its database connections, but regain them after a successful restart of the db services. Given the long startup times of gitlab.service, this actually even reduces the downtime caused by a database restart in many cases.

Reasoning/ why this decoupling is fine:
By design, both redis and postgresql can be hosted on another host than the gitlab service itself. The NixOS module actually supports this for postgresql, but not for redis (as a dependency of sidekiq). Still, Gitlab and Sidekiq are designed to cope with temporary unavailabilities of redis and postgresql and do recover from such situations automatically.

Pl-131811

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[osnyx]

Superseded by NixOS#286532