Replace fmt.Sprintf("%s:%d", ip, port) with net.JoinHostPort() to
properly handle IPv6 addresses that need bracket wrapping (e.g.,
[2606:4700:4700::1111]:53 instead of 2606:4700:4700::1111:53).
Without this fix, configuring IPv6 nameservers causes "too many colons
in address" errors because Go's net.Dial cannot parse the malformed
address string.
Fixes#5601
Related to #4074
Co-authored-by: easonysliu <easonysliu@tencent.com>