Fix some compile warnings (#2919)

* Fix some compile warnings
* Some classes made final
* JPA entities made not serializable

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

hawkbit-repository/hawkbit-repository-jpa-hibernate/src/main/java/org/eclipse/hawkbit/repository/jpa/JpaConfiguration.java

@@ -9,6 +9,7 @@
  */
 package org.eclipse.hawkbit.repository.jpa;
 
+import java.io.Serial;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
@@ -142,6 +143,9 @@ public class JpaConfiguration extends JpaBaseConfiguration {
 
     static class CustomHibernateJpaDialect extends HibernateJpaDialect {
 
+        @Serial
+        private static final long serialVersionUID = 1L;
+
         protected CustomHibernateJpaDialect() {
             super();
             this.setJdbcExceptionTranslator(JpaExceptionTranslator.getTranslator());