Support IPv6 addresses

[keevan]

Currently it fails validation for ipv6 addresses, when they probably should be supported and will become more common going forward.

It does seem to save the value (tested refreshing after settings change), but it likely won't work

EDIT: Identified and reported as a Moodle core bug

- https://tracker.moodle.org/browse/MDL-81461

[brendanheywood]

This is a core bug here:

https://tracker.moodle.org/browse/MDL-81461