Fix: Making Flushable public (dotCMS#33028)

It fixed dotCMS#32784

### Proposed Changes
Weld is trying to generate a proxy for HostWebAPIImpl that also
implements Flushable. However, since Flushable is not public, if the
proxy is created in a different package, it won’t be visible to the
runtime-generated proxy class.

Author: freddyDOTCMS

dotCMS/src/main/java/com/dotmarketing/portlets/contentlet/business/Flushable.java

@@ -4,7 +4,7 @@
  * A Flushable is a API that handle cache and provided public mehod to flush the content away
  * @param <T>
  */
-interface Flushable<T> {
+public interface Flushable<T> {
 
     /**
      * Flush all the cache away