When a Jetty 12 Jenkins test harness is used (i.e., one built from the prototype branch of jenkins-test-harness), the following test fails:

• com.dabsquared.gitlabjenkins.connection.GitLabConnectionConfigSSLTest

This is just a matter of changing the API usage from org.eclipse.jetty.server.handler.AbstractHandler to org.eclipse.jetty.server.Handler.Abstract, as also done in two tests on the prototype branch of Jenkins core (which can be used as inspiration here). Unfortunately, this is not backward compatible with Jetty 10, so the PR will have to remain in draft until we release JTH with Jetty 12 support. Still, we should prepare the PR in advance.