Fix jetty-httpclient-12 testlatestdeps

[jaydeluca]

FutureResponseListener was removed in the latest release of jetty, so updating the test to use a CompletableFuture instead

https://scans.gradle.com/s/ua4dxqhl5oed2

org.junit.platform.launcher.core.DiscoveryIssueException: TestEngine with ID 'junit-jupiter' encountered a critical issue during test discovery:

(1) [ERROR] ClassSelector [className = 'io.opentelemetry.javaagent.instrumentation.jetty.httpclient.v12_0.JettyHttpClient12AgentTest', classLoader = jdk.internal.loader.ClassLoaders$AppClassLoader@76ed5528] resolution failed
    Source: ClassSource [className = 'io.opentelemetry.javaagent.instrumentation.jetty.httpclient.v12_0.JettyHttpClient12AgentTest', filePosition = null]
            at io.opentelemetry.javaagent.instrumentation.jetty.httpclient.v12_0.JettyHttpClient12AgentTest.<no-method>(SourceFile:0)
    Cause: java.lang.NoClassDefFoundError: org/eclipse/jetty/client/FutureResponseListener
	at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
	at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3578)
	at java.base/java.lang.Class.privateGetPublicMethods(Class.java:3603)
	at java.base/java.lang.Class.getMethods…