Use Dependabot (or similar) to check and automatically update Actions versions

[d33bs]

GitHub Actions used within CI workflows often go without updates for some time. This can eventually cause issues with CI see #431 , #432 , or provide less support than possible if a version update were to be applied. Dependabot can be configured to update these automatically, just as it does with other dependencies. See here for more: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot#enabling-dependabot-version-updates-for-actions