SpringBoot Upgrade to Jackson 2.12.1

Comment From: philwebb

Interesting that some classes have been renamed. I wonder if we should try to tolerate older Jackson versions as well?

Comment From: dreis2211

@philwebb See https://github.com/FasterXML/jackson-databind/issues/2715 for the reason behind the renaming.

Comment From: snicoll

@valfirst thank you for making your first contribution to Spring Boot. I've polished to tolerate older Jackson versions.

Comment From: wilkinsona

We subsequently upgraded to 2.12.1 (https://github.com/spring-projects/spring-boot/commit/7b39cfb06133c287994483c3ab90fc521a8217a4). I've retitled this PR so that the release notes are accurate and @valfirst is still credited for their contribution.