OAK-10945: Remove usage of Guava Function interface

[reschke]

This replaces Guava Function with JDK Function.

Notable points:

• rewrote apply() implementations as lambda expressions (fwiw, that makes them compatible with both Function variants)
• when a Guava method needs a Guava Function, the JDK function can be "converted" with ::apply

(next step would be to do the same for Predicates, but I'd like to do this step-by-step)