Change the default branch of the repo from master to main

[tnir]

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Steps to reproduce

1. Access to https://github.com/reactjs/react-rails/blob/master/docs/get-started.md#reactjs-versions

   • 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`

2. Click VERSIONS.md to get 404 error.

Expected behavior

VERSIONS.md should be linked from ocs/get-started.md#reactjs-versions.

Actual behavior

The link causes an 404 error.

Why

See #1327