enable dependabot for github actions

[meeg]

I tried to bring our GitHub actions up to date in #398 but missed some. Should use Dependabot to do this automatically.

• https://stackoverflow.com/questions/70192880/how-to-know-what-version-of-a-github-action-to-use
• https://docs.github.com/en/code-security/how-tos/secure-your-supply-chain/secure-your-dependencies/keeping-your-actions-up-to-date-with-dependabot