mail/mpop: Update to 1.4.20

This release fixes detection of server capabilities, in particular the
list of supported authentication methods: the last method reported by
the server was not recognized by mpop. This bug was introduced in
version 1.4.19.

Changelog:
https://marlam.de/mpop/news/mpop-1-4-20/

PR:	280568
MFH:	2024Q3