date_histogram combined interval field is deprecated

Hi guys it seems https://github.com/elastic/elasticsearch-ruby/blob/ee83caf8ea090775b19190341eeee2cd627fdcd0/elasticsearch-dsl/lib/elasticsearch/dsl/search/aggregations/date_histogram.rb#L41
should be updated according `interval` deprecation https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-datehistogram-aggregation.html

Thanks

Start at elasticsearch-dsl/lib/elasticsearch/dsl/search/aggregations/date_histogram.rb around line 41 and read the linked Elasticsearch date_histogram documentation on interval deprecation. Update the deprecated combined interval field according to that guidance, then confirm the generated date_histogram DSL uses the current field and behavior.

Written by the indexing model from the issue text.