This commit finalizes the work done to rename `size` to `max_docs` in
reindex and update/delete by query. `size` is no longer supported in URL
or outer level body for the 3 APIs.

Continuation of elastic#41894

Closes elastic#24344

Update and delete by query calls from rest-high-level-client generate
size in the body (meaning scroll size). Given that size in body for
these two requests has always meant scroll-size, there is no reason to
tighten that in this PR, so removing those checks. Also, this ensures
that rest-high-level-client requests generated with 7.x codebase works
against 8.0.