[BK] Migrate the kibana-on-merge job (#184391)

## Summary
This PR adds the pipeline resource definition and adjusts the agent
targeting rules for the
[kibana-on-merge](https://buildkite.com/elastic/kibana-on-merge) job.

Migration staging runs: 
-
https://buildkite.com/elastic/kibana-migration-pipeline-staging/builds/116

8.14: #184513 
7.17: #184497

.buildkite/pipeline-resource-definitions/kibana-on-merge.yml

@@ -0,0 +1,49 @@
+# yaml-language-server: $schema=https://gist.githubusercontent.com/elasticmachine/988b80dae436cafea07d9a4a460a011d/raw/rre.schema.json
+apiVersion: backstage.io/v1alpha1
+kind: Resource
+metadata:
+  name: bk-kibana-on-merge
+  description: 'Runs for each commit of Kibana, i.e. each time a PR is merged'
+  links:
+    - url: 'https://buildkite.com/elastic/kibana-on-merge'
+      title: Pipeline link
+spec:
+  type: buildkite-pipeline
+  owner: 'group:kibana-operations'
+  system: buildkite
+  implementation:
+    apiVersion: buildkite.elastic.dev/v1
+    kind: Pipeline
+    metadata:
+      name: kibana / on merge
+      description: 'Runs for each commit of Kibana, i.e. each time a PR is merged'
+    spec:
+      env:
+        SLACK_NOTIFICATIONS_CHANNEL: '#kibana-operations-alerts'
+        GITHUB_BUILD_COMMIT_STATUS_ENABLED: 'true'
+        GITHUB_COMMIT_STATUS_CONTEXT: buildkite/on-merge
+        REPORT_FAILED_TESTS_TO_GITHUB: 'true'
+        ELASTIC_SLACK_NOTIFICATIONS_ENABLED: 'true'
+      allow_rebuilds: true
+      branch_configuration: main 7.17 8.13 8.14
+      default_branch: main
+      repository: elastic/kibana
+      pipeline_file: .buildkite/pipelines/on_merge.yml
+      skip_intermediate_builds: false
+      provider_settings:
+        build_branches: true
+        build_pull_requests: false
+        publish_commit_status: false
+        trigger_mode: code
+        build_tags: false
+        prefix_pull_request_fork_branch_names: false
+        skip_pull_request_builds_for_existing_commits: false
+      teams:
+        everyone:
+          access_level: BUILD_AND_READ
+        kibana-operations:
+          access_level: MANAGE_BUILD_AND_READ
+        appex-qa:
+          access_level: MANAGE_BUILD_AND_READ
+        kibana-tech-leads:
+          access_level: MANAGE_BUILD_AND_READ

.buildkite/pipeline-resource-definitions/locations.yml

@@ -22,6 +22,7 @@ spec:
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-fleet-packages-daily.yml
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-migration-staging.yml
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-on-merge-unsupported-ftrs.yml
+    - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-on-merge.yml
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-performance-daily.yml
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-performance-data-set-extraction-daily.yml
     - https://github.com/elastic/kibana/blob/main/.buildkite/pipeline-resource-definitions/kibana-pr.yml