platform: preserve IPv6 multicast route added by kernel

Kernels < 5.11 add a route like:

  unicast ff00::/8 dev $IFACE proto boot scope global metric 256 pref medium

to allow sending and receiving IPv6 multicast traffic. Ensure it's not removed it when we do a route sync in mode ALL.

In kernel 5.11 there were commits:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ceed9038b2783d14e0422bdc6fd04f70580efb4c https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a826b04303a40d52439aa141035fca5654ccaccd

After those the route looks like

  multicast ff00::/8 dev $IFACE proto kernel metric 256 pref medium

As NM ignores routes with rtm_type multicast, the code in this commit is not needed on newer kernels.

https://bugzilla.redhat.com/show_bug.cgi?id=2004212