Update EOL GitHub Actions dependencies

[jwnrt]

Pull Request Overview

This pull request updates GitHub Actions dependencies which use Node v12 (which will stop working soon) to Node v16.

GitHub will remove Node v12 from the action runners on the 14th of August.

Testing Strategy

All the updated workflows run on: pull_request, so if this PR passes then the CI still works.

I checked the major-version release notes of the updated actions and didn't see any breaking changes apart from updating Node v12 to Node v16.

I see that one workflow is using actions/setup-node so there could be breakage here.

TODO or Help Wanted

There is one action left that hasn't been updated to Node v16 (actions-rs/toolchain) which doesn't seem to be maintained any more. It's used in most workflows.

There's a replacement action called dtolnay/rust-toolchain, but I'm not sure it supports using the toolchain specified in rust-toolchain.

I think GitHub's container comes with rustup, so you could potentially drop this action if the repo had a rust-toolchain.toml which specified the required components. The rustc update script currently depends on this being a plaintext file, so would need updating.

Documentation Updated

• Updated the relevant files in /docs, or no updates are required.

Formatting

• Ran make prepush.

[ppannuto]

This is an easy yes, thanks!

W.r.t. the toolchain version and its action, I'll make a separate PR on that in a little bit — it looks like the new rust-toolchain.toml also supports explicit lists of required components and versions; we currently have a bunch of make machinery that that could displace as well, which is exciting.