Upgrade to GitHub-native Dependabot

[dependabot-preview[bot]]

Now that Dependabot is built in to GitHub and generally available, we're encouraging everyone to migrate to get the best possible Dependabot experience. This pull request migrates your configuration from Dependabot.com to a config file, using the new syntax. When you merge this pull request, we'll swap out dependabot-preview (me) for a new dependabot app, and you'll be all set!

With this change, you'll now use the Dependabot page in GitHub, rather than the Dependabot dashboard, to monitor your version updates. Dependabot is now configured exclusively using config files.

If you've got any questions or feedback for us, please let us know by creating an issue in the dependabot/dependabot-core repository.

Learn more about migrating to GitHub-native Dependabot

Please note that regular @dependabot commands do not work on this pull request.

:robot: :yellow_heart: