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
(cherry picked from commit 222334fa671811658266afc697034089aaaf5eb8)