xen: 4.19.1 -> 4.20.0

[SigmaSquadron]

DO NOT MERGE: Xen 4.20 has not yet been released.

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Everything seems to work, and the build time has been reduced by over half!

We'll continue to maintain 4.19.x in 24.11. This should not be backported.

Things done

• Built on platform(s)
  • x86_64-linux
• 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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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.

^{_{haha, it's the funny weed number!}}

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