use features of Java 21 with `--enable-preview`

.idea/misc.xml

@@ -8,7 +8,7 @@
       </list>
     </option>
   </component>
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_X" default="true" project-jdk-name="21" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_21_PREVIEW" project-jdk-name="21" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>