Change source attribute of Suggestion entity in /api/v2/suggestions back to a string (#29108)

app/serializers/rest/suggestion_serializer.rb

@@ -1,7 +1,20 @@
 # frozen_string_literal: true
 
 class REST::SuggestionSerializer < ActiveModel::Serializer
-  attributes :source
+  attributes :source, :sources
 
   has_one :account, serializer: REST::AccountSerializer
+
+  LEGACY_SOURCE_TYPE_MAP = {
+    featured: 'staff',
+    most_followed: 'global',
+    most_interactions: 'global',
+    # NOTE: Those are not completely accurate, but those are personalized interactions
+    similar_to_recently_followed: 'past_interactions',
+    friends_of_friends: 'past_interactions',
+  }.freeze
+
+  def source
+    LEGACY_SOURCE_TYPE_MAP[object.sources.first]
+  end
 end