Don't allow anonymous users to search with text

If you're an anonymous user and the
`: disable_anonymous_search` flag is turned on,
then we don't allow them to
perform a text search in Issues, Epics, or Merge Requests.

Changelog: changed

app/finders/issuable_finder.rb

@@ -332,6 +332,7 @@ class IssuableFinder
   def by_search(items)
     return items unless search
     return items if items.is_a?(ActiveRecord::NullRelation)
+    return items if Feature.enabled?(:disable_anonymous_search, type: :ops) && current_user.nil?
 
     if use_cte_for_search?
       cte = Gitlab::SQL::CTE.new(klass.table_name, items)

config/feature_flags/ops/disable_anonymous_search.yml

+---
+name: disable_anonymous_search
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70223
+rollout_issue_url:
+milestone: '14.3'
+type: ops
+group: group::project management
+default_enabled: false