switch default branch name to main in docs/ci

[braingram]

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

Unlike spacetelescope/stdatamodels#247 and spacetelescope/stpipe#136 it is likely that downstream packages are referring to the current default branch. For some period of time (I cannot find a statement that define for how long), github will redirect these requests to the renamed branch.

Known downstream uses will be listed here:

• (broken) link in crds: fix jwst schemas link in documentation crds#1021

For developers, they will need to update their local clones and forks to match the new default branch.

Once the issue of updating packages that use the current jwst default branch is addressed, we should be able to:

• coordinate renaming with all interested parties as mentioned by @nden
• 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).

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly