Update default primary server list for the root zone

DEFAULT_IANA_ROOT_ZONE_PRIMARIES should be updated. This was pointed out by @rocharj (thanks!).

AFAICT, only the b.root-servers.net addresses are outdated, but perhaps there are other adjustments that can be made (e.g. adding new addresses providing root zone AXFRs?), so I decided to create an issue for this task.