wwan: enable ipv6 by default in ModemManager backend

In the original addition of the ModemManager backend for mobile
broadband, IPv6 was set to be disabled/ignored by default. The original
motivation for this is not obvious, but it should be gone after 11
years. Some carriers have IPv6-only networks for which the default
IPv4-only connection attempt is inappropriate. Enable IPv6 by default to
support more WWAN networks without special configuration.

Changing the default does not affect IPv4-only support thanks to
fallbacks implemented in nm_modem_get_connection_ip_type().

Link: https://gitlab.com/postmarketOS/pmaports/-/issues/2752
Fixes: a9032724cb ('modem-manager: new `NMModemBroadband'')

src/core/devices/wwan/nm-modem-broadband.c

@@ -918,7 +918,7 @@ complete_connection(NMModem             *modem,
                                   _("GSM connection"),
                                   NULL,
                                   NULL,
-                                  FALSE); /* No IPv6 yet by default */
+                                  TRUE);
 
         return TRUE;
     }
@@ -939,7 +939,7 @@ complete_connection(NMModem             *modem,
                                   _("CDMA connection"),
                                   NULL,
                                   iface,
-                                  FALSE); /* No IPv6 yet by default */
+                                  TRUE);
 
         return TRUE;
     }