Fix MySQL backwards compatibility test failure (#5085)

Sequel::Postgres::Dataset is only defined when the postgres adapter is
loaded. In regular CI, the case_insensitive_string_monkeypatch requires
sequel/adapters/postgres unconditionally, making the constant available
even in MySQL runs. The backwards compatibility tests skip migrations
(NO_DB_MIGRATION=true), so the monkeypatch is never loaded and the
constant is undefined.

Use Sequel::Dataset instead, which is always available and sufficient
for the mock (it only needs .update).

Author: philippthun

spec/unit/jobs/v3/services/update_broker_job_spec.rb

@@ -448,7 +448,7 @@ module V3
 
           context 'when database disconnects during state rollback' do
             let(:catalog_error) { StandardError.new('Catalog fetch failed') }
-            let(:mock_dataset) { instance_double(Sequel::Postgres::Dataset) }
+            let(:mock_dataset) { instance_double(Sequel::Dataset) }
 
             before do
               allow_any_instance_of(VCAP::CloudController::V3::ServiceBrokerCatalogUpdater).to receive(:refresh).and_raise(catalog_error)