fix: require Java SE 21 in the classpath

Author: rubenporras

org.eclipse.lsp4e/.classpath

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <classpath>
-	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER">
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-21">
 		<attributes>
 			<attribute name="maven.pomderived" value="true"/>
 		</attributes>