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-ql-jpa/src/main/java/org/eclipse/hawkbit/ql/QueryException.java

@@ -9,6 +9,8 @@
  */
 package org.eclipse.hawkbit.ql;
 
+import java.io.Serial;
+
 import lombok.EqualsAndHashCode;
 import lombok.Getter;
 import lombok.ToString;
@@ -20,10 +22,13 @@ import lombok.ToString;
 @ToString(callSuper = true)
 public class QueryException extends RuntimeException {
 
+    @Serial
+    private static final long serialVersionUID = 1L;
+
     public enum ErrorCode {
         INVALID_SYNTAX,
         UNSUPPORTED_FIELD,
-        GENERIC // an other
+        GENERIC // another
     }
 
     @Getter