Rebase onto original GitHub Markup gem

I would like to see us rebase this gem on the current version of GitHub Markup, which is currently at version 4.0.1. I think when we fork a gem in order to make somewhat minor changes, that we should work to keep it up to date with the parent, including bug fixes. They have added support for CommonMark as the default, instead of redcarpet, which by itself is, I think, worthy of an upgrade.

Of course we keep our own updates as needed, such as the new timeouts.

I got to looking at this because of PlantUML is not parsing ReSTRUCTURED files (gitlab#354020 - closed). We overrode how the language on code blocks is handled for .rst files, and that no longer aligns with how we treat this in GitLab. It looks like moving back to the original handling would be the right move.