Improve similarity integration.

[jpountz]

This improves the way similarities are plugged in in order to:

• reject the classic similarity on 7.x indices and emit a deprecation
  warning otherwise
• reject unkwown parameters on 7.x indices and emit a deprecation
  warning otherwise

Even though this breaks the plugin API, I'd like to backport to 7.x so
that users can get deprecation warnings when they are doing something
that will become unsupported in the future.

Closes #23208
Closes #29035

[elasticmachine]

Pinging @elastic/es-search-aggs

[jimczi]

Thanks for removing the classic similarity.
Regarding the validation of setting, I wonder if we should not try to change the format for similarity so that we can validate them directly in the settings framework. Since this change targets 7 only I think it's a good time to make such a change.
For instance we could use an affix setting like index.similarity.*.BM25.b and index.similarity.*.BM25.k1 that would not require to add special logic for validation ?

[jpountz]

@jimczi What about digging this idea in a follow-up change?

[jimczi]

We discussed with @jpountz and I understand the intent better now, it's important to have this in 6.x if we want to refuse unknown parameter in 7 so I am fine if we do the validation in the similarity service. We can follow up on this to see how we could fold this validation in the settings framework more naturally.
Also the parent_join tests fail because they still use the classic similarity, you'll need to remove it from these tests ;).