Change ES aggregations to use terms type

What does this MR do and why?

Related to #365052 (closed)

Note: this feature is behind a disabled by default search_blobs_language_aggregation feature flag and is not currently implemented in the frontend (yet)

• Refactor type of Elasticsearch aggregation used to terms aggregation
• Refactor existing specs to support returned json format

Screenshots or screen recordings

old - composite aggregation json response

note: not sorted by count

new - terms aggregation json response

note: sorted by count desc

How to set up and validate locally

1. make sure gdk is setup with elasticsearch enabled and advanced search setup
2. enable the feature flag: Feature.enable(:search_blobs_language_aggregation)
3. hit the new endpoint: http://gdk.test:3000/search/aggregations?scope=blobs&search=*
4. verify it comes back with json

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.