Undo FreeBSD 12.x workaround on IPv6 ifconfig for 12.4 and up

Undo FreeBSD 12.x workaround on IPv6 ifconfig for 12.4 and up

commit 5e19cc2c1bf22d introduced a workaround for a race condition
that showed itself on IPv6 ifconfig on FreeBSD 12.x - sometimes breaking
IPv6 connectivity on tun/tap interfaces.

This was fixed on the FreeBSD side in 12.4, 13.1 and up, and 13.0 is
no longer supported.  So conditionalize the workaround on "12.0..12.3",
to be fully removed later when 12.3 is also running out of support.

v2: fix version number comparison

Trac: 1226

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230107162558.59659-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25911.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>