Update docker.elastic.co/ci-agent-images/eck-region/buildkite-agent Docker tag to v1.15 (#134828)

This PR contains the following updates:

| Package | Update | Change | |---|---|---| |
docker.elastic.co/ci-agent-images/eck-region/buildkite-agent
([changelog](https://redirect.github.com/elastic/ci-agent-images/commits/main/container-images/eck-region/buildkite-agent))
| minor | `1.5` -> `1.15` |

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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"