Migrate to jdk 21 (#2295)

* docker containers using images with jre 21
* project is still building with target java 17 - so jar artifacts are java 17 compatible
* provided option to have different version for client libraries (e.g. ddi and management REST api)

Signed-off-by: Avgustin Marinov <Avgustin.Marinov@bosch.com>

hawkbit-ddi/hawkbit-ddi-api/pom.xml

@@ -23,6 +23,10 @@
     <artifactId>hawkbit-ddi-api</artifactId>
     <name>hawkBit :: DDI :: REST API</name>
 
+    <properties>
+        <java.version>${java.client.version}</java.version>
+    </properties>
+
     <dependencies>
         <dependency>
             <groupId>org.eclipse.hawkbit</groupId>