Upgrade from yaml.v2 to yaml.v3

[cole-miller]

In #230 we switched to using gopkg.in/yaml.v3 directly, where previously we'd been using a module (github.com/ghodss/yaml) that called into yaml.v2. Then in #232 I switched the dependency from yaml.v3 to yaml.v2 because of an apparent bug, #231. We should still upgrade the dependency back to yaml.v3 eventually, possibly after reporting/fixing any upstream issues that affect us.