[Feature flag] Enable cmark CommonMark renderer by default

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 set the feature flag use_cmark_renderer to true, per #345744 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #345744 (closed)