Add missed ref: 6.3.x in verify-staged-artifacts.yml

Test staged artifacts of this branch against Spring Integration `6.3.x` branch

.github/workflows/verify-staged-artifacts.yml

@@ -25,6 +25,7 @@ jobs:
         with:
           repository: spring-projects/spring-integration
           show-progress: false
+          ref: 6.3.x
 
       - name: Set up Gradle
         uses: spring-io/spring-gradle-build-action@v2