Parsedown upgrade to 1.7.5 and switch to fork

The original erusev/parsedown repository has not seen any release since
2019, and the owner does not seem too eager to continue maintaining it.

We are switching to a community-maintained fork 1 which has a PHP 8.4
compatible release.

Fixes 0035217, PR https://github.com/mantisbt/mantisbt/pull/2164