[cd.14] Migration - Document removal of response headers from default undertow configuration

As part of work on WFLY-9947 (https://github.com/wildfly/wildfly/pull/10974/files), there have been removed server headers Server and X-Powered-By which were in default configuration of the server. These changes should be effective since Wildfly 13/EAP EO13.

To reflect this change, we should probably remove section 2.2.5. Removing Undertow Response Headers in our Server Security Guide.

This JIRA is to update the Migration Guide with this information.

• We need to update it with the information about the removal of the response filter headers from the default Undertow configuration.
• We need update a few configuration examples in 4.5. Web Server Configuration Changes.