archivers/py-borgbackup: update to 1.2.4, unbreaking with msgpack 1.0.5

Borgbackup pins the exact, full, patch-level based version range
and the prior borgbackup 1.2.3 wanted to have at most msgpack 1.0.4.

Update to 1.2.4 which allows msgpack 1.0.5, and pin the exact msgpack
version in our depends to "fail early".

And other changes including bug fixes:
https://github.com/borgbackup/borg/blob/1.2.4/docs/changes.rst#version-124-2023-03-24

based on a
PR: 270665
Submitted by: otis@

As in-depth soft robustness measure, add relevant warning comments to
devel/py-msgpack/Makefile, and while there, clean up the python:3.6+
to just a bare python because 3.6 and older are long gone from ports.