BWC mode for saved_objects/_find endpoint required for all versions of 6.x

[spalger]

With #17512, which was backported to 6.3, we removed the GET /api/saved_objects/{type?} endpoint and replaced it with GET /api/saved_objects/_find. This is a breaking change that will cause incompatibility between clients and servers in the process of upgrading to 6.3 or beyond. To prevent from breaking clients during the transition we need to continue supporting the GET /api/saved_objects/{type?} API until 7.0.

[epixa]

These saved object APIs are experimental, so if it is really important that we break them, we can. The only API we need to be extremely careful with is the import API.

That said, breaking APIs just because we can isn't really a great practice in any case. If it's possible to introduce some sort of fallback mechanism or something in 6.x for the old behavior, that certainly would be ideal.

[spalger]

After discussing this a bit further we realized that Kibana clients that are from an old version of Kibana will not be able to use any APIs because of the version checks that the Kibana server does, so this isn't actually an issue.