Feature request: Disallow downgrades

[eclairevoyant]

Upstreams sometimes release tags out of order, for example if there's a stable and LTS version branches that are both getting maintenance. In such cases, running nix-update may downgrade stable to the LTS if the LTS tag is newer. It would be useful to disallow such downgrades in that scenario, and ensure only monotonically increasing version numbers.

Example: https://github.com/NixOS/nixpkgs/pull/334315