Make critical discovery issue severity configurable

[marcphilipp]

Overview

• Remove Severity.DEPRECATION
• Rename Severity.NOTICE to Severity.INFO
• Make critical discovery issue severity configurable
• Delete obsolete test class
• Configure critical severity to INFO for dogfooding

Issue: #242

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes