Rename master branch to main

[mkurz]

@SethTisue and @ihostage already renamed the master branch in some play repos:

• rename master branch to main cachecontrol#163
• rename master branch to main play-file-watch#136
• rename master to main, matching new GitHub default play-json#598
• Use main branch as default instead master play-soap#282
• rename master branch to main play-ws#597
• rename master to main, aligning with GitHub default twirl#416

Now that we are centralising configs in the .github repo it's a good idea that all repos have the same default branch name.

To change to main locally (if the remote to this repo is called upstream):

git branch -m master main # rename master to main (actually "move")
git fetch upstream
git branch -u upstream/main main # set upstream
git remote set-head upstream -a # make sure HEAD is set correctly for this remote (autodetect)