net/rsync: drop py-cmarkgfm build dep, fix inert path substitutions

Drop python and py-cmarkgfm from the BUILD_DEPENDS: The tarball ships
pre-rendered nroff man pages, so the upstream md2man toolchain is not needed.

While here, fix a post-patch REINPLACE_CMD pattern that has been silently
inert since 2022 (PR 263787): Replace with two correct substitutions so the
installed man pages reference ${ETCDIR} instead of /etc/rsync*.

Bump PORTREVISION because installed man page content changes.

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