Update Jackson support to require 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 the following at some point:

• 02ee7fd
• 379b2ce
• 42d0f72

[wilkinsona]

Following some experimentation, it has become apparent that it will be too complex to provide full support for both Jackson 2 and Jackson 3.

We are going to update Boot's Jackson support (auto-configuration, JacksonTest, etc) to require Jackson 3. Limited support for Jackson 2 will remain in the form of dependency management to help users configure their own use of Jackson 2 where it's still needed. It will be possible to do so in an app that's also using Jackson 3 in places, in other words, using Jackson 2 and Jackson 3 side-by-side in the same app is a supported arrangement but the former will have to be configured manually rather than relying upon auto-configuration.