Use to_json call in donation campaigns (#38223)

spec/requests/api/v1/donation_campaigns_spec.rb

@@ -78,7 +78,7 @@ RSpec.describe 'Donation campaigns' do
         end
 
         before do
-          stub_request(:get, "#{api_url}?platform=web&seed=#{seed}&locale=en").to_return(body: Oj.dump(campaign_json), status: 200)
+          stub_request(:get, "#{api_url}?platform=web&seed=#{seed}&locale=en").to_return(body: JSON.generate(campaign_json), status: 200)
         end
 
         it 'returns the expected campaign' do