libnm: avoid serializing "ipv6.addr-gen-mode" default to D-Bus

When serializing setting properties to GVariant/D-Bus, we usually
omit values that are set to the default. That is done by libnm(-core),
so it happens both on the daemon and client side. That might be
useful to avoid a large number of properties on D-Bus.

Before changing the default value for "ipv6.addr-gen-mode" ([1]), we
would not serialize the previous default value ("stable-privacy").
Now we would serialize the new default value ("default). This change
causes problems.

Scenario 1: have a profile in the daemon with "ipv6.addr-gen-mode=stable-privacy",
have an older daemon version before [1] and a newer client after [1]. Result:

  The daemon exposes the profile on D-Bus without the addr-gen-mode
  field (because it's the default). To the client, that is interpreted
  differently, as "ipv6.addr-gen-mode=default". This is already somewhat
  a problem.
  More severe is when modifying the profile, the client would now serialize
  the value "default" on D-Bus, which the older daemon rejects as invalid.
  That means, you could not modify the profile, unless also resetting
  addr-gen-mode to "stable-privacy" or "eui64".

You can imagine other scenarios where either the daemon or the client is
before/after change [1] and the addr-gen-mode is set to either "default"
or "stable-privacy". Depending on what scenario you look, that can either be
good or bad.

Scenario 1 is pretty bad, because it means `dnf upgrade NetworkManager
&& nmcli connection modify ...` will fail (if the daemon was not
restated). So try to fix Scenario 1, by also not serializing the new
default value on D-Bus. Of course, some of the scenarios will get
different problems, by exacerbating one side misunderstanding the actually
set value and interpreting a missing value on D-Bus wrongly.  But those
problems are likely less severe.

In case both client and daemon are older/newer than [1], it doesn't
matter either way. The problem happens with different version and is
caused by a change of the default value.

[1] e6a33c04eb

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1372

src/libnm-core-impl/nm-setting-ip6-config.c

@@ -836,8 +836,7 @@ nm_setting_ip6_config_class_init(NMSettingIP6ConfigClass *klass)
                                              NM_SETTING_IP6_CONFIG_ADDR_GEN_MODE_DEFAULT,
                                              NM_SETTING_PARAM_NONE,
                                              NMSettingIP6ConfigPrivate,
-                                             addr_gen_mode,
-                                             .to_dbus_including_default = TRUE);
+                                             addr_gen_mode);
 
     /**
      * NMSettingIP6Config:token: