Rename actorSpecification in DATASET/INPUT/OUTPUT schemas on platform to avoid confusion with actor.json

[jancurn]

We're using actorSpecification declaration also in these files, but that is confusing, because when you look at the file, you simply can't tell what it is, or google for anything. By replacing it with actorInputSchemaVersion, actorOutputSchemaVersion, or actorDatasetSchemaVersion, the users will intuitively know what the file's content is about, and have something to Google for when they see the file.

This means we need to update our parsers of Dataset and Input schema (keep backwards compatibility!) and update docs, e.g. https://docs.apify.com/platform/actors/development/actor-definition/output-schema#structure