* Upgrade our github actions to "node16"
Each github actions run has a lot of warnings about using node12 so this
upgrades our repository to using node16. I'm hoping no other changes are
needed and I suspect other actions we're using are on node12 and will
need further updates, but this should help pin down what's remaining.
* Update `actions/checkout` workflow to `v3`
* Update to `actions/cache@v3`
* Update to `actions/upload-artifact@v3`
* Drop usage of `actions-rs/toolchain`
* Update to `actions/setup-python@v4`
* Update mdbook version
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