devel/gnupg: update to 2.5.20, latest upstream

The stable branch of GnuPG is the 2.5 series. The 2.4 series is
end-of-life. Release announcement at

https://lists.gnupg.org/pipermail/gnupg-announce/2026q2/000504.html (2.5.19)
https://lists.gnupg.org/pipermail/gnupg-announce/2026q2/000505.html (2.5.20)

Co-installability with security/gnupg1 is removed. FreeBSD ports
include compatibility symlinks gpg2 and gpgsplit2 for compatibility
with the 2.4 series which used the gpg-is-gpg2 configuration.

Note that the manpage says "2.6", even though this is version 2.5.
gpg --version does report the correct version.