Increase minimum supported Guava from 20.x to 22.x for Jackson 2.20

[cowtowncoder]

(note: pre-req for #157)

Although "default" Guava dependency has been increased to 25.x for Jackson 2.16 and later, the minimum supported version is still 20.x.
But time may have come to increase that to 22.x (and eventually higher -- probably 25.x) for Jackson 2.18.

Filing this issue for discussion before 2.18.0-rc1 is released.

[cowtowncoder]

Based on brief discussions we are thinking of this going in 2.19, not 2.18, due to timing.

[cowtowncoder]

Missed the boat with 2.19 so moving up to 2.20 then.