How to identify sample inputs that require Java 9+?

[cpovirk]

Currently, we say that samples must work under Java 8. But this is not a long-term solution.

The answer is probably either "separate directories" or "a comment somewhere in the source file."

We had discussed a little on https://github.com/jspecify/jspecify/pull/122#discussion_r480336166. I'm opening this issue so that we have a place to discuss when we first write a sample that uses var or something (#132).