Issue #4443 - Track backport of ALPN APIs to Java 8.

Continuous integration builds for Jetty 9.3.x are now done with Java 9.
Jetty 9.3.x needs Java 9+ to be built in order to build also the ALPN
modules that depend on Java 9 APIs.
Jetty 9.3.x would not be buildable with OpenJDK 8u252 or later,
but it is an EOL branch and required Java 9 to be built anyway.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

Jenkinsfile

@@ -7,11 +7,11 @@ pipeline {
   stages {
     stage("Parallel Stage") {
       parallel {
-        stage("Build / Test - JDK8") {
+        stage("Build / Test - JDK9") {
           agent { node { label 'linux' } }
           options { timeout(time: 120, unit: 'MINUTES') }
           steps {
-            mavenBuild("jdk8", "install", "maven3", false)
+            mavenBuild("jdk9", "install", "maven3", false)
             // Collect up the jacoco execution results (only on main build)
             jacoco inclusionPattern: '**/org/eclipse/jetty/**/*.class',
                    exclusionPattern: '' +