Migrate to built-in Dependabot

[HonkingGoose]

Dependabot has recently been integrated into GitHub itself proper.

This pull-request removes the old Dependabot-preview configuration, and places the new Dependabot configuration into the .github folder.

Merging this pull-request closes #1356

Benefits:

• No need to have Dependabot-preview installed as repo administrator
• No need to use a third party site anymore for configuration.
• All configuration is now done via the configuration file, so everybody can see what the settings are.

Drawbacks:

• Update schedule is no longer truly live (Best update frequency is now once a day on Mondays trough Fridays).
• All configuration is now done via the configuration file, so everybody can see what the settings are.

References for my changes:
https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/
https://help.github.com/en/github/administering-a-repository/keeping-your-dependencies-updated-automatically

Steps to take after merging:
@ben you can remove the Dependabot-preview app from your GitHub account after merging this pull-request, unless you're also using Dependabot-preview on other private/public repositories.
You can find your installed GitHub apps when you're logged in, and go to: https://github.com/settings/installations

[ben]

✨ Thanks!

[HonkingGoose]

Cheers, thanks for merging in my work! 🎉 ✨