refactor(validate): Fix a few readability/coding style issues.

• Migrate from the deprecated ioutil.ReadFile() to os.ReadFile(). (https://go.dev/doc/go1.16#ioutil)
• Improve error messages produces by the APIVersion() method. E.g. print the actual and expected type.
• Make APIVersion() a method of the Manifest type. Drop the "Get" prefix. (https://google.github.io/styleguide/go/decisions#getters)
• Clearly separate reading the API version and constructing the schema path. (single responsibility)

Issue: team#56 (closed)