Upgrade Java to 21

Change-Id: If0144af39b481ee4984ebeb7449d9af7b1ce7774
diff --git a/pom.xml b/pom.xml
index f5796b3..8d8a4f4 100644
--- a/pom.xml
+++ b/pom.xml
@@ -9,7 +9,7 @@
   <url>https://korap.ids-mannheim.de/</url>
   
   <properties>
-    <java.version>17</java.version>
+    <java.version>21</java.version>
     <jetty.version>11.0.26</jetty.version>
     <jersey.version>4.0.2</jersey.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>