Differentiate between "v14-style" versioning and "v12-style" versioning

[s3lph]

At the moment, the validator merges all version indications from both api and api_compatibility into one, stripping of the leading 0. from api. This leads to a few confusing situations where thinks like the following are considered valid or at least influence which schema versions an endpoint is (erroneously) validated against, possibly showing up twice in the "validated versions" list:

{"api": "0.13", "api_compatibility": ["13", "14", "15"], ...}

{"api": "0.15", "api_compatibility": ["15"], ...}

{"api": "15", "api_compatibility": ["15"], ...}

This PR attempts to resolve this issue and make results a lot clearer by:

• Requiring versions 0.12 and 0.13 to be only indicated in the api field.
• Requiring versions 14 and later to be only indicated in the api_compatibility field.
• Failing validation if support for unknown schema versions is indicated, including wrong use of major vs minor schema version such as 13 or 0.14.