netcup: Modernize record conversion

[TomOnTime]

Dear @kordianbruck. I could use your help. I'm doing upgrades to the code base and I have no way to test your provider. Please run the integration test and let me know the result. How to run integration tests is here: https://docs.dnscontrol.org/developer-info/integration-tests

Summary

• build downloaded records with modern record factories
• replace legacy DNS name expansion with factory normalization
• propagate conversion errors from provider responses

Validation

• ../../bin/is_modern.sh (Steps 1-7 clean)
• go test ./providers/netcup
• bin/generate-all.sh
• go test ./...

[TomOnTime]

@kordianbruck, GitHub could not retain the reviewer request, so I sent you an invitation for triage access to this repository. Would you please accept the invitation and review this provider modernization when you have a chance? Thank you!

[TomOnTime]

FYI: If CAA, TLSA, or SRV fail the integration tests, look at the code around line 99 of providers/netcup/types.go. The commented out code can be brought back in.

[TomOnTime]

I'm merging this. If there are further bugs to fix, please start a new PR or issue. Base any issues off of release_candidate_v5, please. There will be 1-2 release candidates before this code is merged to main.

Sorry for the inconvenience. I really need to get -rc3 out today. Thank you for understanding.

[kordianbruck]

Sorry for the delay, verified today and opened #4698 to get the provider fully fixed.