nixos/system/activation/bootspec: adopt the latest specification

[RaitoBezarius]

Description of changes

This PR adopts the latest specification of RFC-0125 (hopefully the last one).

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/)
• 23.05 Release Notes (or backporting 22.11 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.

[cole-h]

Now that NixOS/rfcs#125 has been merged, is it time to mark this ready for review? :)

I also believe that there was talk about making enforcement to this spec disabled by default (so as not to have Go in the closure by default). Should that be implemented here, or in a separate PR? (Or is it already implemented?)

[RaitoBezarius]

Superseded by #224489 ! Everything is done in that PR :)