Revert "max_score and _score are required (and can be null)"

[pquentin]

Reverts #1144

I spent hours getting Kibana type checking to work with this change, but it's just too much work:

• There are thousands of tests that use mock search responses, they all need to be changed to _score, max_score or both.
• Some code is taking advantage of the fact that the get response and search hits have the same required fields.
• Some Kibana types are extending the search hits.

Since making those types required offers little benefit, let's revert the change.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.