Deprecate the markdown module

See https://forum.xwiki.org/t/parsers-and-serializers-refactoring/17322

The markdown package should be replaced by a better integrated HTML serializer using uniast.

Tiptap is dependent on parts of this module. Since we would rather not invert time maintaining Tiptap, the easiest thing to do is to simply wait for Tiptap to be removed from our code base before removing markdown.