Add buildkite pipeline for updating serverless commit

.buildkite/promote-es-serverless.yml

@@ -0,0 +1,8 @@
+steps:
+  - trigger: elasticsearch-serverless
+    label: ":elasticsearch: Run serverless tests"
+    build:
+      message: "Elasticsearch submodule promotion build"
+      env:
+        ELASTICSEARCH_SUBMODULE_COMMIT: "${BUILDKITE_COMMIT}"
+