HV-924 Making sure AnimalSniffer works on JDK 8

> I would prefer to have this information about which classes not to check as part of the plugin config. Similar to checkstyle. There are also alternative approaches with checkstyle, but it gives you at least the choice.

Note that it could also be specific methods, I guess that'd become a bit cumbersome in a config file. Tbh. I don't really mind either way. The annotation is not even accessible at runtime, so I don't really see any problem.

> WDYT about RequiresJava8 or TargetsJava8 something like that.

We can do that, though it would be slightly wrong. The semantics of the annotation really is "the annotated element does not work on the configured JDK baseline", i.e. "does not run on Java 6" in our case. Theoretically we also might have code which only runs on Java 7 or later, for that the suggested names wouldn't really fit.