This commit enables HTTP/2 support for Tomcat and Undertow, for both
Servlet-based and Reactive applications.

Enabling the `server.http2.enabled` configuration flag is enough with
Undertow.

Tomcat has a few prerequisites:

* Tomcat 8.5 requires JDK8+ and the proper libtcnative version installed
on the host
* Tomcat 9.0.x requires JDK9+

Closes gh-10043