prepare for rename to main

[braingram]

This PR updates the CI and docs to refer to a renamed "main" branch.

As no downstream package appears to reference a specific branch for stdatamodels (and instead uses the default branch) we should be able to:

• rename the default branch to "main"
• merge this PR

It's possible codecov may need to be updated to use "main" (if it's not using the default branch).

Github does quite a number of helpful things when the branch is renamed (see https://github.com/github/renaming#renaming-existing-branches) so if acceptable I would propose we rename the branch, merge this PR, and deal with any fallout (although I do not expect any).

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:exclamation: No coverage uploaded for pull request base (master@1f2acf6). Click here to learn what that means.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #247 +/- ## ========================================= Coverage ? 64.84% ========================================= Files ? 103 Lines ? 5694 Branches ? 0 ========================================= Hits ? 3692 Misses ? 2002 Partials ? 0 ```

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