Motivation for this change

The default gnucash was changed from version 2.4 to 3.1, which requires manual intervention to upgrade successfully. See #39616

This change adds to the NixOS release notes accordingly.

Things done

• Built the NixOS manual with cd nixpkgs; nix-build -I nixpkgs=$PWD nixos/release.nix -A manual.x86_64-linux
• Visually verified the generated documentation:

• Tested the link in the doc to ensure it goes to the right place.
• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• [ ] Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.