[scikit-learn] Renaming the default branch to `main`

[Adrin]

GitHub now supports renaming <https://github.com/github/renaming> the
default branch with this done automatically:

Renaming a branch will:

   - Re-target any open pull requests
   - Update any draft releases based on the branch
   - Move any branch protection rules that explicitly reference the old name
   - Update the branch used to build GitHub Pages, if applicable
   - Show a notice to repository contributors, maintainers, and admins on
   the repository homepage with instructions to update local copies of the
   repository
   - Show a notice to contributors who git push to the old branch
   - Redirect web requests for the old branch name to the new branch name
   - Return a "Moved Permanently" response in API requests for the old
   branch name


We have talked in this issue
<https://github.com/scikit-learn/scikit-learn/issues/17595> about renaming
the branch, but since this is a major change, hence this email to engage
and inform the broader community.

Cheers,
Adrin
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