Remove input fields from exported LTR models

[afoucret]

Context:

In Elasticsearch >= 8.15.0, we update the PUT _ml/trained_models/<model_id> API for LTR models and specifying input.field_names is not supported anymore.

Previously, the field was required and eland was putting feature names into it as a hack.

The Elasticsearch PR: https://github.com/elastic/elasticsearch/pull/109437

PR content:

The behavior now depends of the es version and the input.field_names is set only for ES < 8.15.0

[afoucret]

@elasticmachine rerun buildkite/eland

[davidkyle]

I'd really like to get rid of the required input.field_names for uploading HuggingFace models too, your ES PR contains has a clever solution I might extend for the HF NLP models