Concierge framework does not support extensions in JDK11

[JochenHiller]

When running Eclipse SmartHome packaging sample (https://github.com/eclipse/smarthome-packaging-sample) using Concierge 5.1 with JDK11, it will complain when installing bundle "slf4j-jdk14-1.7.22.jar with this message:

Concierge> Exception in thread "main" org.osgi.framework.BundleException: Framework does not support extensions
	at org.eclipse.concierge.compat.service.BundleManifestTwo.processManifest(BundleManifestTwo.java:227)
	at org.eclipse.concierge.BundleImpl$Revision.<init>(BundleImpl.java:1579)
	at org.eclipse.concierge.BundleImpl$JarBundleRevision.<init>(BundleImpl.java:3587)
	at org.eclipse.concierge.BundleImpl.readAndProcessInputStream(BundleImpl.java:395)
	at org.eclipse.concierge.BundleImpl.<init>(BundleImpl.java:225)
	at org.eclipse.concierge.Concierge.installNewBundle(Concierge.java:4078)
	at org.eclipse.concierge.Concierge.installNewBundle(Concierge.java:4041)
	at org.eclipse.concierge.Concierge$BundleContextImpl.installBundle(Concierge.java:4662)
	at org.eclipse.concierge.compat.service.XargsFileLauncher.processXargsInputStream(XargsFileLauncher.java:189)
	at org.eclipse.concierge.compat.service.XargsFileLauncher.processXargsFile(XargsFileLauncher.java:83)
	at org.eclipse.concierge.Concierge.doMain(Concierge.java:606)
	at org.eclipse.concierge.Concierge.main(Concierge.java:525)

The check if JVM supports extensions in https://github.com/eclipse/concierge/blob/master/framework/org.eclipse.concierge/src/org/eclipse/concierge/Concierge.java#L927 seems not to work in JDK11.