ARIA main branches renamed

Following a particularly exasperated ping from JamesN :) I've changed 
the name of the primary branch in several ARIA repositories from 
"master" to "main". This has been done in:

  * aria
  * core-aam
  * accname
  * dpub-aria
  * dpub-aam
  * graphics-aria
  * graphics-aam
  * svg-aam
  * css-aam

Because of uncertain ownership or coordination needs, I have *not* done 
it for:

  * html-aam
  * aria-practices
  * aria-at

Pull requests have automatically been updated to target the "main" 
branch. You should fetch into your local clones to pick up this update.

Since the time I first experimented with this, GitHub has made the 
process easier. But I have a nagging feeling I'm forgetting a step. Let 
me know if you run into problems from this change.

Michael