Disallow disabling `_field_names`

[jpountz]

Now that _field_names has basically no overhead anymore (#26930), we should ignore the enabled setting in 6.x (and log deprecation) and reject it in 7.x, or maybe 8,x if we want to maintain compatibility for templates.

cc @colings86

[colings86]

Caveat to this: _fieldNames has basically no overhead in the case where doc_values and norms are enabled. If doc_values are disabled then _field_names will have the same overhead as before.
I wonder if for a high index through put use case you would still want the option to disable _field_names because you are disabling doc_values on fields that you don't want to aggregate to increase indexing throughput and don't want to counter this by adding the overhead of writing to _field_names?

[javanna]

@elastic/es-search-aggs

[ppf2]

As part of this ticket, can we also clarify the documentation to mention that there is much less overhead now for most use cases except for the caveats mentioned above? Thx!