checkstyle: bump to 6.18
Also, forgot to simplify the checkstyle setup in the extension after we moved from an ant folder hierarchy to a gradle one. Do that now.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Also, forgot to simplify the checkstyle setup in the extension after we moved from an ant folder hierarchy to a gradle one. Do that now.