fixing dns server reference

[arne21a]

#346

PR Checklist

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• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• can't test because my setup uses different keys. Logic works with my keys in my setup.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Description

March platform release deploys two firewalls:
Hub attached firewall connectivity_secure_firewalls_env
VNet attached firewall as a central DNS instance connectivity_private_dns_firewalls_env

This change adds to correct firewall as dns server of the newly created VNet.
See the linked issue

Does this introduce a breaking change

• YES

• NO

It will change the current DNS Setting of existing ASVM Subscriptions. In my opinion, this setting is correct.

Testing

Build a level0 to level3 landingzone based on the march release, then use the asvm to create a hub attached subscription.
In my modified scenario, my change worked fine.

[LaurentLesle]

LGTM