Update docker.elastic.co/ci-agent-images/eck-region/buildkite-agent Docker tag to v1.15

Author: elastic-renovate-prod[bot]

.buildkite/pipelines/pull-request/pr-upgrade.yml

@@ -2,5 +2,5 @@ steps:
   - label: pr-upgrade
     command: ".buildkite/scripts/run-pr-upgrade-tests.sh"
     agents:
-      image: "docker.elastic.co/ci-agent-images/eck-region/buildkite-agent:1.5"
+      image: "docker.elastic.co/ci-agent-images/eck-region/buildkite-agent:1.15"
       memory: "4G"