fix: Do not report java.util.ConcurrentModificationException

[pboos]

The internally used library sometimes reports a violation like the following:

OpenAPI spec validation error [validation.response.body.schema.unknownError]
POST https://domain.com/v1/test
User Agent: ???
Response Status Code: 200
INFO - An error occurred during schema validation - com.google.common.util.concurrent.UncheckedExecutionException: java.util.ConcurrentModificationException.: []

As this is not a real violation, it should not be reported as such. Therefore adding it to be excluded.