[Feature flag] Enable cmark CommonMark renderer (`:use_cmark_renderer`)

What does this MR do and why?

Originally we used the Ruby version, which gave us additional flexibility to handle a special case.

We've now refactored that out and can use the C version, which is much more performant over the Ruby version.

This MR removes the feature flag use_cmark_renderer. It has been enabled in production since Nov 18, 2021.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #345744 (closed)