Revert "[TASK] Upgrade CodeMirror to v.3.21"

This reverts commit 12f8576aa7c52f51ce3aa1839508d86501014681.

Fixes: #55341
Change-Id: I5a5c9a5b3da67e3d2f708f647fba8e80e0d1418d
Reviewed-on: https://review.typo3.org/27059
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers