Always sort Elasticsearch searches by relevance not updated_at

**TL;DR** Elasticsearch defaults to sorting by `_score` and this is what
we want so remove all the explicit sorting.

At present some searches are being sorted by `_score` (ie. the relevance
score) while others are being first sorted by `updated_at` then
`_score`.

There doesn't really appear to be any good reason to use `updated_at` as
the primary sort followed by `_score` since this will bury the most
relevant matches behind lots of recent matches. Using sort in this way
will only ever sort by `_score` if the `updated_at` was equal but this
is basically never going to happen since these are timestamps so
`_score` in practice is never factoring in to the sorting which probably
was never the intent here.

Since the [default sorting is by
score](https://www.elastic.co/guide/en/elasticsearch/guide/master/sorting.html)
we can leave it out of all queries.

We can in future look into whether or not we want recency to factor into
the sorting using more advanced tools like
[`function_score`](https://www.elastic.co/guide/en/elasticsearch/guide/current/function-score-query.html)
but for now sorting just by relevance is better than sorting just by
`updated_at`.

ee/changelogs/unreleased/228575-sort-by-relevance-not-updated-at.yml

+---
+title: Always sort Elasticsearch searches by relevance not updated_at
+merge_request: 36774
+author:
+type: fixed

ee/lib/elastic/latest/application_class_proxy.rb

@@ -71,11 +71,6 @@ module Elastic
             }
           end
 
-        query_hash[:sort] = [
-          { updated_at: { order: :desc } },
-          :_score
-        ]
-
         query_hash[:highlight] = highlight_options(fields)
 
         query_hash

ee/lib/elastic/latest/git_class_proxy.rb

@@ -92,8 +92,6 @@ module Elastic
 
         options[:order] = :default if options[:order].blank?
 
-        query_hash[:sort] = [:_score]
-
         res = search(query_hash, options)
         {
           results: res.results,
@@ -154,8 +152,6 @@ module Elastic
 
         options[:order] = :default if options[:order].blank?
 
-        query_hash[:sort] = [:_score]
-
         if options[:highlight]
           query_hash[:highlight] = {
             pre_tags: ["gitlabelasticsearch→"],

ee/lib/elastic/latest/note_class_proxy.rb

@@ -16,11 +16,6 @@ module Elastic
         query_hash = project_ids_filter(query_hash, options)
         query_hash = confidentiality_filter(query_hash, options[:current_user])
 
-        query_hash[:sort] = [
-          { updated_at: { order: :desc } },
-          :_score
-        ]
-
         query_hash[:highlight] = highlight_options(options[:in])
 
         search(query_hash, options)

ee/lib/elastic/latest/project_class_proxy.rb

@@ -42,8 +42,6 @@ module Elastic
 
         query_hash[:query][:bool][:filter] = filters
 
-        query_hash[:sort] = [:_score]
-
         search(query_hash, options)
       end
     end

ee/spec/lib/gitlab/elastic/snippet_search_results_spec.rb

@@ -22,8 +22,11 @@ RSpec.describe Gitlab::Elastic::SnippetSearchResults, :elastic, :sidekiq_might_n
     end
 
     it 'returns the correct page of results' do
-      expect(results.objects('snippet_titles', page: 1, per_page: 1)).to eq([snippet2])
-      expect(results.objects('snippet_titles', page: 2, per_page: 1)).to eq([snippet])
+      # `snippet` is more relevant than `snippet2` (hence first in order) due
+      # to having a shorter title that exactly matches the query and also due
+      # to having a description that matches the query.
+      expect(results.objects('snippet_titles', page: 1, per_page: 1)).to eq([snippet])
+      expect(results.objects('snippet_titles', page: 2, per_page: 1)).to eq([snippet2])
     end
 
     it 'returns the correct number of results for one page' do