Remove prefer_v2_templates flag on indexing operations

[dakrone]

After careful decision we decided to remove the ?prefer_v2_templates flag introduced in #55411, instead we will consider addition of a V2 index template opting into the newer behavior.

To that end, we need to remove all the cases of the prefer_v2_templates query string parameter and its effect from the 7.8+ code.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Indices APIs)