Add back a metadata key for api minimum version

Fixes an oversight: people are going to upgrade from 5 to 6, some will
have 3rd party plasmoids from the store that will still be for plasma5.

We want a different error message when we know the plasmoid is "old" In
Palsma4 times we used to have for the pure JavaScript plasmoids an api
version key that could be used for things like that.

This makes it a bit more future proof for any future api change there
will be

CCBUG:474456