Upgrade Guava to a more recent version
Upgrade Guava to a more recent version

There was no particular reason to use 20.0, and that version has been
released back in 2016.

RoboZonky/robozonky,RoboZonky/robozonky,triceo/zonkybot,triceo/robozonky,triceo/robozonky,triceo/zonkybot