Version pinning for GHA actions

[straight-shoota]

So I have this PR for upgrading the versions of actions running in GHA: #12501

For the Windows jobs we have pinned specific commits instead of versions (ref #11477 (comment)). I understand that this is more secure because the author of the action could technically move the tag to a different commit, right? And there's no lockfile to keep the versions pinned.

My question is, is this really necessary? (evidence suggests that it is a recommended safety practice: https://julienrenaux.fr/2019/12/20/github-actions-security-risk/, https://michaelheap.com/ensure-github-actions-pinned-sha/)
And shouldn't we do this everywhere then?

The problem with commit hashes is that it's not clear what version they represent. So that makes it a bit difficult to manage and understand. Maybe there are tools which make this easier? (for example https://github.com/mheap/pin-github-action automatically adds comments referring tot he version number).

We should probably look into a tool for automating action upgrades (that's regardless of using versions or hashes).

[caspiano]

Dependabot is very easy to set up, but it doesn't support bumping digests.
Renovatebot natively supports updating Github Action dependencies' digests, so that may be the way to go.

[straight-shoota]

Sounds good. I just checked that out on one of my repos.
Here is the PR that transforms versions into hashes: straight-shoota/crinja#54

[oprypin]

I think it's good to specify exact hashes for not-so-trustworthy sources. It looks nice to me what renovatebot does.