Fix bad settings for ipv6

With my FAI, i get following during connect to APN

connection error message: IPv6 only allowed

on /etc/NetworkManager/system-connections/*.nmconnection

When i change

[ipv6]
addr-gen-mode=stable-privary
method=ignore

to

[ipv6]
addr-gen-mode=stable-privacy
method=auto

It caused by modem and autodetect not ask to set ipv6 method to auto so it's automatically set it to ignore.

My patch force to set method to automatic. It works well with my FAI and OP6.

Dbus pcap before patch dbus.pcap

Dbus pcap after patch dbus.pcap

It can be backported to Plasma/6.1

Fix potentially #349 (closed)