gnome.updateScript: Add more type annotations
gnome.updateScript: Add more type annotations

Also switch to enums instead of string values and extract the main function.

Validated with:

    nix-shell -I "nixpkgs=channel:nixos-unstable" -p python3 -p python3Packages.mypy -p python3.pkgs.libversion -p python3.pkgs.requests -p python3.pkgs.types-requests --run 'python -m mypy pkgs/desktops/gnome/find-latest-version.py'

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