checkstyle: 6.19 -> 8.8

Semi-automatic update. 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 8.8 with grep in /nix/store/09g6vrj90r6lbwj2hwb319sgd3ppp14k-checkstyle-8.8
- found 8.8 in filename of file in /nix/store/09g6vrj90r6lbwj2hwb319sgd3ppp14k-checkstyle-8.8