[Java][Spring] Resolve serialization warning from RFC3339DateFormat (#6960)

Adds `serialVersionUID` to RFC3339DateFormat.java to avoid Java warning.

samples/server/petstore/springboot-delegate/src/main/java/io/swagger/RFC3339DateFormat.java

@@ -9,6 +9,8 @@ import java.util.Date;
 
 public class RFC3339DateFormat extends ISO8601DateFormat {
 
+  private static final long serialVersionUID = 1L;
+
   // Same as ISO8601DateFormat but serializing milliseconds.
   @Override
   public StringBuffer format(Date date, StringBuffer toAppendTo, FieldPosition fieldPosition) {