[migration] stop caching the mapping from constraint type name to eid
It's not so frequent that a cache seems necessary, and we were not
invalidating that cache when adding a new constraint type.
Related to #3724892.
[RichString] Add markdown support
Supporting markdown requires the 'Markdown' python packages, and since it
widely available on the target platforms adding it as a strong requirements
will not be a big constraint.
Closes #3814302