nixos/version: Require a stateVersion

[dasJ]

22.05 shows a warning for this, so we can safely remove it now. Users
were warned.

Closes #65314

Description of changes

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/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[pennae]

there are a bunch more instances of eval-config being run all over the nixos tree, eg in the gnome installer and a bunch of hydra jobs. those will all need adjusting as well. not sure that's all though, there may be hidden instances lurking.

[roberth]

Was going to suggest throwing a custom error in default, but that will obfuscate which system is missing this info. The module system's built-in message includes the entire attribute path.
(This is assuming NixOS gets hooked into a deployment tool's module system correctly; can't vouch for each of them, but NixOps has an open PR that will accomplish this.)

[roberth]

22.05 shows a warning for this, so we can safely remove it now.

It's only been merged for less than a month though #124261 (comment)
Not sure how often nixos-unstable users generally update, but it seems a bit short.