Switch default branch from master to main

[edmorley]

As part of ensuring inclusive language is used throughout all Heroku repositories, we should switch from the current default branch of master to main.

This will involve:

• Creating a new branch main, based off current master
• Changing the default branch in GitHub to main
• Updating the branch protection settings
• Updating the target branch of any open PRs
• Updating any in-repo code/docs references (such as the Travis badge) -> #172
• Checking that there are no branch-specific Travis environment variables set
• Updating the weekly Travis cron to build from the new branch
• Checking that Travis still works -> https://travis-ci.com/github/heroku/stack-images/builds/178267673
• Checking for any other deep links to this repository in other repos that include the branch name
• Deleting the master branch

@W-7837260@

[edmorley]

All done here :-)