Update CI

[dbaynard]

Resolves #63

• Update github actions
• Add dependabot to keep actions up to date
• Fix set-output deprecation
• Replace abandoned rust toolchain action(s)
• Replace deprecated upload-release-asset action

[dbaynard]

Further updates are nontrivial — I'll have to come back to this.

[brandur]

Awesome, thanks! I allowed CI to go through and it's working.

And actually, if you want to remove this from review, I'll merge it. IMO better to at least make some incremental progress on getting this build fixed up. We can continue chipping away at the other stuff in other PRs.