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) -> Replace 'master' branch references with 'main' #422
• Checking that there are no branch-specific Travis environment variables set
• Checking that Travis still works -> https://travis-ci.com/github/heroku/heroku-buildpack-go/builds/178287407
• Checking for any other deep links to this repository in other repos that include the branch name
• Update branch-specific URL on https://devcenter.heroku.com/articles/go-support
• Deleting the master branch

Note:
Most users will be consuming this buildpack via the buildpack-registry stable release (using the shorthand alias heroku/go). Some will be using the master branch via a buildpack URL of https://github.com/heroku/heroku-buildpack-go, and others will be pinned to a specific branch or tag using https://github.com/heroku/heroku-buildpack-go#some-branch-or-tag.

In rare cases people may be using the latter form to specify the master branch, even though it's redundant to specify the branch name. Such usages will need to be updated to use either the default-branch GitHub form, or else the shorthand form (recommended).

Also GitHub now redirects any web links (such as the /blob/ URLs) whose branch doesn't exist, which reduces the urgency of catching every stray docs reference:
https://github.blog/changelog/2020-07-17-links-to-deleted-branches-now-redirect-to-the-default-branch/

@W-7869571@

[edmorley]

In rare cases people may be using the latter form to specify the master branch

Analysing buildpack URLs used (slack), I found only 1 app using the #master form with this buildpack and it hasn't had a deploy since 2018, so we should be good to just delete the old master branch.