Revert change to Java 21

Author: dfuchss

pom.xml

@@ -75,7 +75,7 @@
     <ardoco.version>${revision}</ardoco.version>
 
     <project.source.encoding>UTF-8</project.source.encoding>
-    <java.version>21</java.version>
+    <java.version>17</java.version>
     <maven.compiler.source>${java.version}</maven.compiler.source>
     <maven.compiler.target>${java.version}</maven.compiler.target>
     <maven.compiler.release>${java.version}</maven.compiler.release>