Load date fields through 'fields' instead of 'docvalue_fields'.

[jtibshirani]

Currently the Discover search loads the document _source, and also loads date fields using docvalue_fields in order to apply the right formatting. Doc values are column-oriented and are more expensive to retrieve, so Elasticsearch places a limit on the number of fields that can be retrieved (index.max_docvalue_fields_search). If a user has more date fields than this limit, the Discover search can fail.

Elasticsearch merged a new fields option that loads fields from _source, and supports date formatting. Discover could switch to loading dates through fields instead of docvalue_fields to prevent hitting the limit.

This would resolve #22897. Also relates to #69049.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

Adding geo tag since maps also loads date fields via doc_values.

[kertal]

For this change SearchSource has to support fields, so adding AppArch

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[lukeelmers]

Somehow I had missed this issue until now -- I'm planning to address this as a part of the work for #77241.