Issue #6980 mark the field as accessible (#6981)

Signed-off-by: Olivier Lamy <oliver.lamy@gmail.com>

jetty-servlet/src/main/java/org/eclipse/jetty/servlet/listener/ELContextCleaner.java

@@ -52,6 +52,8 @@ public class ELContextCleaner implements ServletContextListener
             //Get a reference via reflection to the properties field which is holding class references
             Field field = getField(beanELResolver);
 
+            field.setAccessible(true);
+
             //Get rid of references
             purgeEntries(field);