mikrotik: Modernize record conversion

[TomOnTime]

Dear @hedger. 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 the modern record factories
• pass the domain config through conversion for modern label handling
• modernize provider unit-test record setup while preserving custom forwarder behavior

Validation

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