Only use a single yaml library.

Created by: mrnugget

We currently use yaml.v3 in internal/campaigns because it supports parsing YAML into a map[string]interface{}, whereas yaml.v2 only supports parsing into a map[interface{}]interface{} and the latter can't be marshalled into JSON.

The only other place where we use YAML is in cmd/src/validate.go and I don't see any reason why that can't use yaml.v3.