Use a more recent version of guava

The model-test framework uses maven-resolver-provider which transitively pulls in a quite old version of guava, which:

1) Is different from what full uses (although it's just a testsuite dep in core, so not a big deal)
2) Gets flagged by vulnerability scanners, which is annoying for a test dep.

So see if managing this version in dependencyManagement to the same version used in full works.