[libfabric-users] renaming libfabric master branch to main

[Hefty, Sean]

The master branch has been renamed to main.  For those with a github account who have forked the repo, you should see additional instructions for updating locally (also shown below).

Hopefully CI everywhere doesn't break.  :)

- Sean 

> There have been a few request to rename the libfabric master branch to main, with
> discussion in the ofiwg.  Github provides tools to help assist with the rename, and in
> theory the migration should be painless for most users.
> 
> I plan on handling the rename early tomorrow (Tuesday the 8th), so any problems can be
> responded to.
> 
> Open PRs should update automatically.  Local repos can be updated using the following
> sequence of commands:
> 
> git branch -m master main
> git fetch origin
> git branch -u origin/main main
> 
> - Sean