atk: 2.26.1 -> 2.28.1

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.28.1 with grep in /nix/store/gqr774xsr2qkb7g7s5zz56lyvdf0qm0c-atk-2.28.1
- found 2.28.1 in filename of file in /nix/store/gqr774xsr2qkb7g7s5zz56lyvdf0qm0c-atk-2.28.1