Extend CI intake pipeline to run BC upgrade tests

.buildkite/scripts/get-bc-commit.sh

@@ -0,0 +1,51 @@
+#!/bin/bash
+
+#
+# Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+# or more contributor license agreements. Licensed under the "Elastic License
+# 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+# Public License v 1"; you may not use this file except in compliance with, at
+# your election, the "Elastic License 2.0", the "GNU Affero General Public
+# License v3.0 only", or the "Server Side Public License, v 1".
+#
+
+set -euo pipefail
+
+#TODO: all current branches? Only the "last" one? (9.0 in this case?)
+#source .buildkite/scripts/branches.sh
+#for BRANCH in "${BRANCHES[@]}"; do
+
+BRANCH=9.0
+
+MANIFEST="$(curl -s https://artifacts.elastic.co/releases/TfEVhiaBGqR64ie0g0r0uUwNAbEQMu1Z/future-releases/stack.json |
+jq ".releases[] |
+select(.branch == \"$BRANCH\") |
+select(.active_release == true) |
+.build_candidates |
+to_entries |
+sort_by(.value.completed_at) |
+last |
+.value.manifest_url")"
+
+BC_COMMIT_HASH="$(eval "curl -s $MANIFEST" | jq .projects.elasticsearch.commit_hash)"
+
+echo "steps:
+  - group: bc
+    steps:
+      - label: $BRANCH / bc-bwc
+        command: .ci/scripts/run-gradle.sh -Dbwc.checkout.align=true -Dorg.elasticsearch.build.cache.push=true -Dignore.tests.seed -Dscan.capture-file-fingerprints -Dtests.bwc.main.version=9.1.0 -Dtests.bwc.refspec.main=$BC_COMMIT_HASH -Dtests.jvm.argline=\"-Des.serverless_transport=true\"
+        timeout_in_minutes: 300
+        agents:
+          provider: gcp
+          image: family/elasticsearch-ubuntu-2004
+          machineType: n1-standard-32
+          buildDirectory: /dev/shm/bk
+          preemptible: true
+        retry:
+          automatic:
+            - exit_status: \"-1\"
+              limit: 3
+              signal_reason: none
+            - signal_reason: agent_stop
+              limit: 3
+"