cmark: 0.27.1 -> 0.28.3

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/cmark/versions.

These checks were done:

- built on NixOS
- ran ‘/nix/store/ws8c9r62c7wsas7kj1v0svfs57d53dvv-cmark-0.28.3/bin/cmark -h’ got 0 exit code
- ran ‘/nix/store/ws8c9r62c7wsas7kj1v0svfs57d53dvv-cmark-0.28.3/bin/cmark --help’ got 0 exit code
- found 0.28.3 with grep in /nix/store/ws8c9r62c7wsas7kj1v0svfs57d53dvv-cmark-0.28.3
- directory tree listing: https://gist.github.com/890d356ed9766b2fca77baebee346a0e