Keep GitHub Actions up to date with GitHub's Dependabot

[cclauss]

This will automate the creation of pull requests like

• casey/just#1871

That will autofix #13 and similar issues when Actions are updated in the future.

• 13

Docs:

• 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