eclipsecs-sevntu-plugin is broken since eclipse-cs 8.36.1

Identified at https://github.com/sevntu-checkstyle/sevntu.checkstyle/issues/857 ,

It seems eclipsecs-sevntu-plugin's old meta data XML files no longer work, even though eclipse-cs still has code to support it. This is causing none of Sevntu's checks to appear in the `Checkstyle Configuration` dialog.
https://github.com/checkstyle/eclipse-cs/blob/100aaca6027895c5b1562f4546ee618e04b8a731/net.sf.eclipsecs.core/src/net/sf/eclipsecs/core/config/meta/MetadataFactory.java#L568

Installing a version of eclipse-cs on or before 8.35 seems to resolve the issue and bring back the functionality. This seems to lead to https://github.com/checkstyle/eclipse-cs/issues/256 to be the culprit.

There appears to be 2 issues so far:
1) eclipse-cs doesn't allow any other 3rd party groups to be added except "Other"
2) The presence of the YML file prevents violations from coming out.