[JSTEP-12]: Head branch renaming: (2.19->2.x; master->3.x)

[Tatu Saloranta]

Quick note: I decided that due to the upcoming 3.0.0 release (still
out by a few months but coming), we need to figure out a way to handle
2 (not just 1) Git head branches.
I outline this as JSTEP-12:

https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-12

But the basic idea is that we will have 2 head branches:

* 3.x (renamed "master"), from which the _next_ 3.x version will be
cut. Maintenance branches will then be "3.0", "3.1" and so on
* 2.x (created AFTER 2.19.0 or first RC for it), from which the next
2.x version will be cut.

The first task I have is to start renaming "master" branches as "3.x"
before 3.0.0-rc3 is released, and changing Github CI actions to match.
This mostly concerns Cascading rebuilds.

I intend to get the renaming of master->3.x done within a week -- it
should not have significant impact on others but may be initially
confusing.

Feedback, concerns, questions welcome as usual: either via this
thread, or 3.0.0-rc3 Discussion:

https://github.com/FasterXML/jackson-future-ideas/discussions/82

-+ Tatu +-