Jackson version alignment with Gradle 6

[cowtowncoder]

Same thing as

FasterXML/jackson-databind#2726

with just minor complications:

1. Since we do not extend jackson-base, need to include full plug-in definition
2. 2.x is fine wrt project-version to jackson-bom but 3.x needs care as annotations will only have "major.minor" version, but base has "major.minor.patch"