Minor polish to the "bump CLI" automation

[shati-patel]

We took @tjgurwara99's new bump-cli workflow for its first test run in #1995, which worked great! 🙏🏽 🎉

This PR polished a few minor things noticed during testing: 💅🏽

• Tell Prettier to ignore supported_cli_versions.json when linting/formatting
• Open the generated PR as a draft: CI doesn't run on PRs opened by the github-actions bot, so we need a human to manually prod them. Our existing approach (e.g. in the releases workflow) is to open the bot PR as a draft, and then run CI when a human marks it as ready-for-review.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.