Documentation: Deprecate network pinger

In a meeting on 9/30/19, Engineering and Support agreed to deprecate the network pinger, because some customers are mistaken in thinking it can protect them from split brain. Hence, they are at risk of message loss. Moving the network pinger to a deprecated status means that Red Hat will still support existing deployments that use this, but strongly recommend against using it for new deployments.

The documentation needs to make customers aware of this change in the following ways:

• Add a Deprecated Feature note to the Broker 7.5 Release Notes Describe what the deprecated status means. Inform users that the feature will be removed in a (non-specified) future release. Handled by https://issues.jboss.org/browse/ENTMQBR-2726.
• Add a note to the Migration Guide
• Remove network pinger content from the config guide