Rename "master" branch as "3.x" (JSTEP-12)

[cowtowncoder]

As part of JSTEP-12, we are renaming current "master" branch as "3.x", in preparation of 3.0.0 release, and need for dual head branches.
I have update other repos both wrt renaming and changes to Github Actions (CI), but I don't know this module well enough to make all changes.

So: it'd be great if Scala module could follow same convention (unless @pjfanning strongly disagrees -- renaming is not 100% required for things to work).

Note that this is not an urgent thing to do, just something to get eventually done.

I will also file separate issue for creation of "2.x" branch after 2.19.0 release (one can be created earlier but has no value before 2.19.0 is out).

[pjfanning]

done

[cowtowncoder]

Thank you @pjfanning !