Consider how best to support Jackson 3

[philwebb]

With spring-projects/spring-framework@2a29e16 we'll need to consider how and when we want to support Jackson 3.

Specifically we need to decide if we're to support Jackson 2 and Jackson 3 at the same time, and if we do which should take priority when.

[philwebb]

In the meantime I've suppressed all warnings. We'll need to revert 02ee7fd at some point.