Design doc: go/flutter-update-packages-autoroller

I believe the steps are something like:

1. Settings -> Developer settings -> Personal Access Tokens -> Generate new token
2. Token must have scopes: "repo", "read:org".
3. "Generate token"
4. Copy the access token and store in valentine (this will not be accessible again from GitHub!)

https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/creating-a-personal-access-token

To generate an access token, we may need to create a bot github account (or leverage the engine autoroll account https://github.com/engine-flutter-autoroll): https://stackoverflow.com/questions/31159275/how-do-i-generate-the-github-oauth-token-for-organization-accounts