Downgrade Azure provider "Failed to extract" log message to debug

[bittrance]

The only thing present in the external-dns when running with the Azure provider is:

time="2021-03-19T21:11:37Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:12:37Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:13:38Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:14:38Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:15:39Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:16:39Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:17:40Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."
time="2021-03-19T21:18:47Z" level=error msg="Failed to extract targets for 'example.com' with type 'NS'."

It is not really an error to have an NS record in one's zone. Nor is external-dns expected to act on it. Looking at the Azure private provider

external-dns/provider/azure/azure_private_dns.go

Line 117 in 030d86c

log.Debugf("Failed to extract targets for '%s' with type '%s'.", name, recordType)

, this message is debug, which seems more reasonable. This PR changes the Azure public provider to match.

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[Raffo]

/lgtm
/approve

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