v1.24.6

Revert migration to the `mdka` crate

The reverted change was introduced in v1.24.5. Unfortunately everyday usage
shows, that the `mdka` crate is not as mature as the former solution with
the `fast_html2md` crate.

Otherwise, no new features. The `extensions` table of the `zettel` scheme was
updated. This concerns only users of the `zettel` scheme _not_ using Markdown.