Updated validation for DNS DMARC creation/update fixes #6575

Merged Pyte C requested to merge Pyte/ispconfig3:6575-fix-dns-dmarc-validation into develop

Before the query checked if there is a TXT record that starts with 'v=DKIM' and has '._domainkey' in the name to validate that a DKIM record exists as DMARC needs it. Now we check either for the TXT record as before or if there is a CNAME record that has '._domainkey' in the name as it is possible that we only direct towards the record within another zone entierly.

Closes #6575 (closed)

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