[TLS-446] Update pipeline to use gpctl (#160508)

## Summary
This PR enables gpctl for kibana with the config definition on
serverless gitops:
https://github.com/elastic/serverless-gitops/blob/main/gen/gpctl/kibana/config.yaml

This will enable treating kibana as a service and not as a stack
component, but this will only happen on dev and only after merging:
https://github.com/elastic/serverless-gitops/pull/306

So practically for now nothing else changes. We just need the extra step
to onboard the new worfklow. Another PR will succeed this one after
verifying that everything works as expected, that will remove the
existing (legacy) step that is using a bash script instead of gpctl for
QA and Staging.

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

@@ -83,9 +83,18 @@ echo "--- Trigger image tag update"
 if [[ "$BUILDKITE_BRANCH" == "$KIBANA_BASE_BRANCH" ]]; then
   cat << EOF | buildkite-agent pipeline upload
 steps:
+  - label: ":argo: Update kibana image tag for kibana-controller using gpctl"
+    async: true
+    branches: main
+    trigger: gpctl-promote
+    build:
+      env:
+        SERVICE_COMMIT_HASH: $GIT_ABBREV_COMMIT
+        REMOTE_SERVICE_CONFIG: https://raw.githubusercontent.com/elastic/serverless-gitops/main/gen/gpctl/kibana/config.yaml
+
   - trigger: serverless-gitops-update-stack-image-tag
     async: true
-    label: ":argo: Update image tag for Kibana"
+    label: ":argo: Update image tag for Kibana using the legacy script (used in QA/Staging)"
     branches: main
     build:
       env: