gh-34999: Enable dependabot for github action updates
Enables automatic PRs from dependabot that update the versions of github
actions that we use in our workflow. Thus, we no longer need to do this
manually.
More details: https://docs.github.com/en/code-
security/dependabot/dependabot-version-updates/about-dependabot-version-
updates
URL: https://github.com/sagemath/sage/pull/34999
Reported by: Tobias Diez
Reviewer(s): Matthias Köppe
