[CI] Set workflow branch to 5.0

Author: mnocon

.github/workflows/backend-ci.yaml

@@ -94,4 +94,4 @@ jobs:
 
   integration-tests:
       name: "REST integration tests"
-      uses: ibexa/rest/.github/workflows/integration-tests-callable.yaml@main
+      uses: ibexa/rest/.github/workflows/integration-tests-callable.yaml@5.0