HV-924 Making sure AnimalSniffer works on JDK 8

> Whyt not?

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.

> The ignores parameter applies to the classes to be ignored, say Optional. You'd then get no warning whereever that type is used, but e.g. we want to fail the build when using Optional in say ValidatorImpl. You only get that level of granularity with the annotation approach.

I understand. I find this a bit odd though. I would have expected that `ignores` is just the alternative to using annotations. Not saying that the current functionality of `ignores` does not make sense, but that yet another option.

> Any alternative proposal?

WDYT about `RequiresJava8` or `TargetsJava8` something like that.