nixos/pam: replace apparmor warnings with assertions

[uninsane]

this warning was introduced here with the intention to upgrade it to an assertion after everyone who hits this has had time to fix their config.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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.

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[uninsane]

marking as "draft" to signal that this probably should be merged right away since it breaks eval for anyone who hasn't tended to their warnings yet. but the changeset should be fine, so whenever anyone else feels it's appropriate, feel free to undraft it on my behalf & it should be good to go.

[uninsane]

i think the 3 weeks it's been is enough of a deprecation notice for unstable users: un-drafting this.