Make changes to the spec to indicate new platform/buildpack API versioning scheme

[sambhav]

This is blocking the experimental features related to dockerfiles release.

[natalieparellano]

Update: we decided not to ship Dockerfiles in a pre-release API. We do want to update the specs to indicate that CNB spec versions can contain pre-release identifiers (so that consumers can update their parsing logic), but we should first confirm that we're okay adopting our own versioning schema as proper semver includes a patch number.