Add validation for setting a new userMaximum

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

src/main/java/mops/gruppen2/controller/WebController.java

@@ -327,6 +327,7 @@ public class WebController {
                                 @RequestParam("group_id") String groupId,
                                 KeycloakAuthenticationToken token) {
         Account account = keyCloakService.createAccountFromPrincipal(token);
+        validationService.checkIfNewMaximumIsValid(maximum, groupId);
         controllerService.updateMaxUser(account, UUID.fromString(groupId), maximum);
         return "redirect:/gruppen2/details/members/" + groupId;
     }