Prior to this commit, the HTTP/2 server auto-configuration for Jetty
would require Conscrypt as a hard dependency.

This commit updates the auto-configuration for more flexibility and now
allows the following deployments:

* JDK9+ with the JDK ALPN implementation
* JDK8u252+ with the backported ALPN implementation
* Conscrypt with no JDK requirement

The auto-configuration now improves detection and guides developers in
case there is a missing `jetty-alpn-*-server` dependency.

The reference docs in the HOWTO section has been updated accordingly.

Closes gh-22188