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)

[zacharyburnett]

LGTM!

[nden]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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