Fix getAllGroupWithVisibilityPublic method

Co-Authored-By: andibuls <andibuls@users.noreply.github.com>
Co-Authored-By: tomvahl <tomvahl@users.noreply.github.com>
Co-Authored-By: Talha Caliskan <killerber4t@users.noreply.github.com>

src/main/java/mops/gruppen2/service/GroupService.java

@@ -86,6 +86,7 @@ public class GroupService {
         return visibleGroups.parallelStream()
                             .filter(group -> group.getType() != null)
                             .filter(group -> !userGroupIds.contains(group.getId()))
+                            .filter(group -> group.getVisibility() == Visibility.PUBLIC)
                             .collect(Collectors.toList());
 
     }