[Maintenance] Change PMD to only report errors for production code

[devtobi]

Relevant component

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Problem description (optional)

Currently PMD throws errors in test code as well.
As test code does not require as a high quality as code in production, ershould change the behaviour of PMD
to only fail the build for errors in production code and throw warnings for problems in test code (if that is possible)

Desired solution

Adjust the PMD configuration to only throw errors for production code and throw warnings for test code.

Considered alternatives (optional)

Alternatively modify the rule set for test scenarios to an acceptable subset.

Additional context (optional)

No response

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