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

[docs/jetty/README.md](https://github.com/docker-library/docs/blob/master/jetty/README.md#http2-support) 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.