chore: fix file paths and add missing newlines

backend/build.gradle.kts

@@ -6,7 +6,7 @@ plugins {
 }
 
 checkstyle {
-    configFile = file("$rootDir/config/checkstyle.xml")
+    configFile = file("$rootDir/config/checkstyle/checkstyle.xml")
 }
 
 tasks.withType<Checkstyle> {

backend/src/main/java/de/szut/casino/config/OpenAPIConfiguration.java

@@ -58,4 +58,4 @@ public class OpenAPIConfiguration {
     }
 
 
-}
+}

backend/src/main/java/de/szut/casino/security/KeycloakLogoutHandler.java

@@ -45,4 +45,4 @@ public class KeycloakLogoutHandler implements LogoutHandler {
         }
     }
 
-}
+}