Quick note: default branches change from 2.x to 3.x (for non-deprecated Jackson repos)

[Tatu Saloranta]

So: with the recent 3.0.0 GA release (see
https://github.com/FasterXML/jackson/wiki/Jackson-Release-3.0,
https://cowtowncoder.medium.com/jackson-3-0-0-ga-released-1f669cda529a
), we will be switching focus so that:

1. Jackson 2.x branch will focus more on bug fixes, stability,
reducing risk (some new features added, low-risk)
2. Jackson 3.x will be the place for all new features, future plans

Although the majority of users are (and probably will remain for near
future) on 2.x versions, development focus will be on 3.x. And as such
I think it makes sense to change default branches of all Jackson repos
to 3.x -- excluding deprecated repos that do not have such branch (but
only 2.x).

-+ Tatu +-