Update Go and toml-test toolchain in CI

Hey 👋🏻 Dependabot bumped `rtoml` for me in one of my projects and I got nosey and decided to look through the repo, I noticed you're using a very old version of `actions/setup-go` and of the toml-test repo (now under the `toml-lang` org rather than `BurntSushi`) so I've updated them in this PR. I've obviously made the assumption that you _want_ newer versions and weren't pinning for some specific reason so feel free to close this if that's the case 🙂