Java 15: IllegalAccessError when using MP Rest Client

[andymc12]

There is an MP Rest Client failure when running with Java 15:

testRetryOnceOnFailThenSucceed:junit.framework.AssertionFailedError: 2020-10-15-15:38:21:772 Expected response 200 received 500 (Internal Server Error) while connecting to http://localhost:8010/retryApp/RetryTestServlet?testMethod=testRetryOnceOnFailThenSucceed. 
Error stream: Error 500: java.lang.Exception: java.lang.IllegalAccessError: class org.apache.cxf.microprofile.client.MicroProfileClientProviderFactory$$Lambda$286/0x00000008011c3e68 tried to access protected method 
'int org.apache.cxf.jaxrs.provider.ProviderFactory.compareCustomStatus(org.apache.cxf.jaxrs.model.ProviderInfo org.apache.cxf.jaxrs.model.ProviderInfo)' 
(org.apache.cxf.microprofile.client.MicroProfileClientProviderFactory$$Lambda$286/0x00000008011c3e68 is in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @45ebd1d8&#59; 
org.apache.cxf.jaxrs.provider.ProviderFactory is in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @6131a468)

Marking this as a release bug because users of previous versions of Liberty may run into this if they run with Java 15 (though I think Java 15 is not yet supported on those Liberty versions).