Change severity in Checkstyle output to lowercase

[JannikArndt]

Hi! 👋

I'm currently adding this GitHub Action to our pipeline to display and annotate the code with the results from the scapegoat-run. Unfortunately, everything is just an "Annotation", because the GH Action expects the severity in the xml to be lowercase.

The DTD unfortunately doesn't define the severity attribute (https://checkstyle.org/dtds/configuration_1_3.dtd), but

• the google_checks use warning as default: https://github.com/checkstyle/checkstyle/blob/master/src/main/resources/google_checks.xml#L22
• the puppycrawl-implementation explicitly changes to lowercase: https://github.com/checkstyle/checkstyle/blob/master/src/main/java/com/puppycrawl/tools/checkstyle/api/SeverityLevel.java#L56
• the sun_checks use error as default: https://github.com/checkstyle/checkstyle/blob/master/src/main/resources/sun_checks.xml#L42
• the docs list all four levels in lowercase: https://checkstyle.sourceforge.io/property_types.html#SeverityLevel

So I think this change would make the output "more compliant" and make parsing the xml a but easier 😊