prepare ci/docs for renaming default branch 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 stpipe (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).

xref: spacetelescope/stdatamodels#247

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6cc6214) 59.15% compared to head (87fff8c) 59.15%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #136   +/-   ##
=======================================
  Coverage   59.15%   59.15%           
=======================================
  Files          24       24           
  Lines        1611     1611           
=======================================
  Hits          953      953           
  Misses        658      658           

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