Make Redcarpet Markdown renderer thread-safe

The Redcarpet library is not thread-safe as described in
https://github.com/vmg/redcarpet/issues/570. Since we instantiate
the Redcarpet renderer in a class variable, multiple Sidekiq threads
can access the work buffer and corrupt the state. We work around
this issue by memoizing the renderer on a thread basis.

Closes #36637
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