This change replaces the fields parameter with stored_fields when it makes sense.
This is dictated by the renaming we made in #18943 for the search API.
The following list of endpoint has been changed to use `stored_fields` instead of `fields`:
* get
* mget
* explain
The documentation and the rest API spec has been updated to cope with the changes for the following APIs:
* delete_by_query
* get
* mget
* explain
The `fields` parameter has been deprecated for the following APIs (it is replaced by _source filtering):
* update: the fields are extracted from the _source directly.
* bulk: the fields parameter is used but fields are extracted from the source directly so it is allowed to have non-stored fields.
Some APIs still have the `fields` parameter for various reasons:
* cat.fielddata: the fields paramaters relates to the fielddata fields that should be printed.
* indices.clear_cache: used to indicate which fielddata fields should be cleared.
* indices.get_field_mapping: used to filter fields in the mapping.
* indices.stats: get stats on fields (stored or not stored).
* termvectors: fields are retrieved from the stored fields if possible and extracted from the _source otherwise.
* mtermvectors:
* nodes.stats: the fields parameter is used to concatenate completion_fields and fielddata_fields so it's not related to stored_fields at all.
Fixes#20155
Jim Ferenczi
Daniel Mitterdorfer
Daniel Mitterdorfer