Enable dependabot checks for CI actions

It would be nice to have [dependabot](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates) configured to periodically check the version of actions we use in our CI, so we can keep them up-to-date with little issue.

Read the linked Dependabot documentation, then inspect the repository's CI action workflows to identify the actions whose versions should be checked. Add the appropriate Dependabot configuration and verify that it targets GitHub Actions updates as requested.

Written by the indexing model from the issue text.