Switch to upstream external-dns

[ytsarev]

• Currently, we maintain a fork of https://github.com/k8gb-io/external-dns. This imposes maintenance costs and security risks of stale dependencies within an image, as we rebuild it rarely.
• The main goal of the fork was to unblock external-dns usage with NS record support.
• We need to evaluate the switch back to the original https://github.com/kubernetes-sigs/external-dns after the project evolved.
• It looks like it supports NS names for Azure and other providers out of the box https://github.com/kubernetes-sigs/external-dns/blob/master/provider/azure/azure.go#L195 and https://github.com/kubernetes-sigs/external-dns/blob/master/provider/recordfilter.go#L21-L23

// SupportedRecordType returns true only for supported record types.
// Currently A, AAAA, CNAME, SRV, TXT and NS record types are supported.
func SupportedRecordType(recordType string) bool {
	switch recordType {
	case "A", "AAAA", "CNAME", "SRV", "TXT", "NS":
		return true
	default:
		return false
	}
}