textproc/md4c: Update 0.5.2.20240225 => 0.5.3, take maintainership

Changelog:
https://github.com/mity/md4c/blob/release-0.5.3/CHANGELOG.md

Commit log:
https://github.com/mity/md4c/compare/481fbfb...release-0.5.3

PR: 294864
Approved by: Henrik Rosenke <rosenke@dssgmbh.de> (former maintainer, via xing.com profile)
Sponsored by: UNIS Labs
MFH: 2026Q2