Add branch protection for "Execute Notebooks" and "API Checks"

[Eric-Arellano]

It's currrently possible to merge on red. This is particularly dangerous because the button to "merge when ready" will merge even if "API Checks" and "Execute Notebooks" are red. This is because we didn't set up those CI checks in branch protection due to issues with how we skip those jobs - when skipped, it would block the PR from merging since it never runs.

To fix, implement https://www.pantsbuild.org/blog/2022/10/10/skipping-github-actions-jobs-without-breaking-branch-protection.