jetty/README.md: Obsolete information about HTTP/2 vs OpenJDK 9 availability

[PerMildner]

docs/jetty/README.md says "Once OpenJDK 9 becomes generally available with built-in support for ALPN, this image will be updated".

This needs updating since OpenJDK 9 has now been generally available for a long time.

[tianon]

cc jetty image maintainers: @gregw @lachlan-roberts @olamy @joakime