Update dependency Earmark to latest version

[RobertDober]

Unfortunately projects depending on PhoenixMarkdown and ex_doc (or EarmarkParser directly) are suffering from some difficulties described here.

This is a little bit unfortunate and I repeat here that indeed Earmark's dependency on ex_doc should be removed.
This is however a major undergoing and updating the dependency to 1.4.30 of Earmark is a good idea anyway, given that <=1.3 is not maintained and 1.5 is ante portas (for quite some while, but well Hannibal did not conquer Rome either 😅 )

[RobertDober]

BTW I just released a version, 1.4.41 1.4.43 of Earmark which avoids this problem in the future by not depending on EarmarkParser anymore.