Migrate CircleCI account mode to “GitHub OAuth app”

[jotaen4tinypilot]

CircleCI seems to support having multiple YAML config files, which might be a way to break up our monolithic configuration into more manageable pieces:

Within CircleCI, a project can have one or more configurations, which are pipeline definitions. Configurations include, but are not limited to, a .circleci/config.yml file in your repository.

This is only supported in Github App mode, not in GitHub OAuth app mode, which we are currently using – see the top-level info box on the aforementioned support document:

GitHub authorization with CircleCI is changing. Starting August 2023 when you authorize your CircleCI account with GitHub, you may find this will be done through our GitHub App, rather than the GitHub OAuth app. You can see which account type you have by heading to the CircleCI web app, select Dashboard from the sidebar, and inspect the URL in your browser:

• This style of URL indicates you authenticated with the GitHub App: https://app.circleci.com/pipelines/circleci/UTxCZDiJ9MLGLC8hR1ZDmg

• This style of URL indicates you authenticated with the GitHub OAuth app: https://app.circleci.com/pipelines/github/<your GitHub username/organization name>

For more information about the differences, see the GitHub docs comparison page.