Keep GitHub Actions up to date with GitHub's Dependabot

[cclauss]

This will automate the creation of pull requests like

• https://github.com/pennersr/django-allauth/pull/3615

And fixes the warnings like at the bottom of https://github.com/encode/django-rest-framework/actions/runs/8079252103

• https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem

@carltongibson Your review, please.

[auvipy]

Not sure if it's super important atm. But will look into it

[cclauss]

Fixes the warning at the bottom right of https://github.com/encode/django-rest-framework/actions/runs/8172879907