Deprecate all version-related API endpoints

[aermakov-zalando]

• Mark endpoints as deprecated in api.yaml
• Make all list endpoints return an empty list, entity endpoints return 404, create/update endpoints do nothing and return 200
• Add metrics for every deprecated endpoint for easier usage tracking

Note that no data is modified in the DB, so we could roll back if necessary. Also, I'm not quite sure if we should return 200 from create operations and then return 404 on reads of supposedly created entities, this might lead to bugs that'd be very hard to diagnose.

[harti2006]

:+1:

[hjacobs]

:+1: