`_doc` should work as an alias for the actual type on 6.7

[jpountz]

Mixed clusters raised an issue with the move to typeless APIs as some internal services such as machine learning need to keep interacting with their internal index during rolling upgrades.

As a consequence moving to typeless APIs in 7.x would break rolling upgrades since 6.7 nodes won't understand these calls. We agreed to treat _doc as an alias of the actual type of the index on 6.7 for the get, index, update, delete and bulk APIs to prevent this issue.

Relates #39018

[elasticmachine]

Pinging @elastic/es-distributed

[jpountz]

Closed via #39505.