Jetty with Conscrypt no longer a requirement for easy HTTP/2

[joakime]

Starting with Java 8u252 (note: not 8u251) the ALPN layer has been backported from Java 11 back to Java 8.

The mandated use of Conscrypt with Jetty for HTTP/2 (in SpringBoot) is now no longer a requirement.
You can use Jetty and HTTP/2 without the need for a bootclasspath or an agent, as long as you use Java 8u252 or newer.

There is also a lack of documentation about how to disable the Conscrypt provider when using Jetty with SpringBoot.

[bclozel]

I'm scheduling this as a documentation improvement for this section of the reference documentation. Depending on the outcome of this, we might also repurpose this issue as an enhancement for removing the conscrypt requirement.

[sunng87]

By the way it's not just a doc change. The Jetty's SslServerCustomizer still requires conscrypt as SSL provider. The conscrypt library has serious memory leak issue. So it would be nice to use JDK based provider or at least configurable.

[bclozel]

We've improved the auto-configuration to be more flexible and allow several deployments: JDK9+, JDK8u252+ or Conscrypt. We're also now guiding developers if there's a missing jetty-alpn-*-server dependency. We've updated the documentation accordingly.