chore: switch default branch to main

[G-Rath]

Summary

This is the canonical default branch these days - might as well get the switch over with; once this is merged ideally the master branch should be renamed to main using GitHub's UI so that workflow runs and other such logs are retained.

Pull Request checklist

Remove this line after checking all the items here. If the item is not applicable to the PR, both check it out and wrap it by ~.

• Add/update test to cover these changes
• Update documentation
• Update CHANGELOG file

[tnir]

@justin808 This code broke the link from /docs/get-started.md to /VERSIONS.md as the default branch is still master. Any idea on changing the default branch from master to main?

react-rails/docs/get-started.md

Line 338 in 6e811b6

Be sure to restart your Rails server after changing these files. See [VERSIONS.md](https://github.com/reactjs/react-rails/blob/main/VERSIONS.md) to learn which version of React.js is included with your `react-rails` version. In some edge cases you may need to bust the sprockets cache with `rake tmp:clear`

[tnir]

See #1349