chore(deps): update buildkite plugin monorepo-diff to v1.5.1

Author: elastic-renovate-prod[bot]

.buildkite/pipeline.yml

@@ -3,7 +3,7 @@
 steps:
   - label: "Triggering pipelines"
     plugins:
-      monorepo-diff#v1.4.0:
+      monorepo-diff#v1.5.1:
         diff: ".buildkite/diff ${BUILDKITE_COMMIT}"
         wait: true
         watch: