DashboardVariables: Use Combobox behind feature flag (#98261)

* Add feature toggle

* Use feature toggle

* Remove usage of renderWithCombobox

packages/grafana-data/src/types/featureToggles.gen.ts

@@ -248,6 +248,7 @@ export interface FeatureToggles {
   improvedExternalSessionHandlingSAML?: boolean;
   teamHttpHeadersMimir?: boolean;
   ABTestFeatureToggleA?: boolean;
+  templateVariablesUsesCombobox?: boolean;
   ABTestFeatureToggleB?: boolean;
   queryLibraryDashboards?: boolean;
   grafanaAdvisor?: boolean;