Motivation for this change

Upgrades Hydra to the latest master/flake branch. To perform this
upgrade, it's needed to do a non-trivial db-migration which provides a
massive performance-improvement[1].

The basic ideas behind multi-step upgrades of services between NixOS versions
have been gathered already[2]. For further context it's recommended to
read this first.

Basically, the following steps are needed:

• Upgrade to a non-breaking version of Hydra with the db-changes
  (columns are still nullable here). If system.stateVersion is set to
  something older than 20.03, the package will be selected
  automatically, otherwise pkgs.hydra-migration needs to be used.

• Run hydra-backfill-ids on the server.

• Deploy either pkgs.hydra-unstable (for Hydra master) or
  pkgs.hydra-flakes (for flakes-support) to activate the optimization.

The steps are also documented in the release-notes and in the module
using warnings.

pkgs.hydra has been removed as latest Hydra doesn't compile with
pkgs.nixStable and to ensure a graceful migration using the newly
introduced packages.

To verify the approach, a simple vm-test has been added which verifies
the migration steps.

[1] NixOS/hydra#711
[2] #82353 (comment)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.