AO3-6410 Re-enable dependabot for Github Actions workflow updates.

[tickinginstant]

Issue

https://otwarchive.atlassian.net/browse/AO3-6410

Purpose

• Restores the .github/dependabot.yml file originally created in AO3-6410 Let dependabot update workflow actions. #4359 and then removed again in AO3-6410 Revert "AO3-6410 Let dependabot update workflow actions." #4368. We had to remove it because it was submitting PRs to all forks with no way to disable it, but that behavior has since been fixed.
• Changes the actions/cache version number to be v3 instead of v3.0.11. In Github Actions, the convention is to have the major version numbers like v3 be updated every time a minor v3.x.y version is released. This means that by setting your version number to a major version, you automatically get rolling updates, and don't have to spend as much time updating things. We originally were using major version updates for everything, but when we temporarily enabled dependabot back in October, the dependabot pull request AO3-6411 Bump actions/cache from 2 to 3.0.11 #4364 changed the version number from major to minor. This was a bug, and has since been fixed, but I wanted to make sure we weren't permanently stuck on minor versions.

Testing Instructions

I'm not sure whether the workflows actually need updates at this point, so this might be difficult to test. Is it enough that we know it worked the first time around? Or should I deliberately lower the version number of one of the actions to test it?

[sarken]

Oh, yay, I wasn't expecting that to get fixed so soon! I think we're good without bumping anything's version number down to test.