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
   • https://github.com/reactjs/react-rails/blob/6e811b6bc3634112a269821ee2f5b512167d39ae/docs/get-started.md#L338
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