hdrop: fix Nix meta.license value

[Silveere]

Description of changes

The agpl3 license was split into agpl3Only and agpl3Plus in nixpkgs, and agpl3 was removed. I've updated it to agpl3Only. See NixOS/nixpkgs#297173.

Things done

• For new programs

  • [ ] Add the program to the README table, with yourself as the maintainer
  • [ ] Add a README for the program itself
  • [ ] Add Makefile (and Nix derivation optionally, otherwise @fufexan will do it)
  • [ ] If the program is a script, add it to the CI checks matrix

• For changes

  • [x] Add changes to the CHANGELOG