fix: adjust onKeycloakEvent listener condition

[LukasLohoff]

Description

Removes the USER_GROUP_MEMBERSHIP_CHANGED condition. When a user changes group membership that should not require the user to be created in shogun.

Currently this results in conflicts with other event listeners.

@terrestris/devs Please review

Pull request type

• Bugfix
• Feature
• Dependency updates
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe)

Do you introduce a breaking change?

• Yes
• No

Checklist

• I understand and agree that the changes in this PR will be licensed under the
  Apache Licence Version 2.0.
• I have followed the guidelines for contributing.
• The proposed change fits to the content of the code of conduct.
• I have added or updated tests and documentation, and the test suite passes (run mvn test locally).
• I have added a screenshot/screencast to illustrate the visual output of my update.