Add verification for Java 8 runtime in CI

It would be great to add some runtime verification w/ Java 8 (not compile, just runtime) to ensure this is still good w/ JDK8. We do not currently have samples to run though so not sure what the best approach here would be.

_Originally posted by @onobc in https://github.com/apache/pulsar-client-reactive/pull/181#discussion_r1711867159_