Revert "[ci] Make container image tag update trigger async (#152970)"

This reverts commit 3b20df4.

This pipeline was made async to decrease the number of build failure
alerts during a stabilization period.  This re-synchronizes the
pipeline.

.buildkite/scripts/steps/artifacts/docker_image.sh

@@ -85,7 +85,6 @@ if [[ "$BUILDKITE_BRANCH" == "$KIBANA_BASE_BRANCH" ]]; then
   cat << EOF | buildkite-agent pipeline upload
 steps:
   - trigger: serverless-gitops-update-stack-image-tag
-    async: true
     label: ":argo: Update image tag for Kibana"
     branches: main
     build: