Figure out when to require Java 11 for new Guava versions

Probably not soon :) I just had another note I wanted to make about this, so now seems like the time to start collecting those notes.

• Since we're a dependency of so many other libraries (including ecosystems like Google Cloud), we aren't likely to drop support for Java 8 until such libraries do. Hopefully they won't take too long, given the relatively small amount of support provided for Java 8 nowadays from libraries and JDK vendors.
• Merely requiring a newer javac to build Guava could bring us some of the advantages of this without most of the downsides.
• But it would be even more convenient not to have to put Java 9+ features behind runtime checks of any kind.
  • [edit: also for VarHandle and for ProcessHandle in #6634]
• And it might eventually be nice to be able to put Java 9+ APIs (or other language features) in our public API. (We can't do that today, even with multi-release jars [edit: well, at least not without isolating any such new APIs to separate classes. edit edit: Actually, the precise behavior in that case is more complicated than I've realized, and it may or may not even be fully specified].)