Update Guava to 20.0.0

[elharo]

21 requires Java 8, so stick with 20

[garrettjonesgoogle]

@ejona86 , I haven't looked over the surface of this repo yet - what is the rule for exposing Guava from google-http-java-client?

If Guava is exposed to consumers, the 17.0 -> 20.0 upgrade will be somewhat disruptive; they are more than 2 years apart, meaning that code against the earlier version may break when the runtime is the later version.

[ejona86]

20 broke a lot of gRPC users, and due to some Precondition overloads added to 20 users can't force a lower version. gRPC decided to downgrade to 19 for the moment to allow time for the libraries to catch up.

I'd suggest updating to 19 here as well.

[mattwhisenhunt]

Fixed in #380.

[garrettjonesgoogle]

For those checking after the fact, we settled with Guava 19.0.