iserver-test: 9.1.10 -> 9.2.4 BC upgrade failure

[ericywl]

The upgrade failed: https://github.com/elastic/apm-server/actions/runs/20839235277/attempts/3

Plan change failed: Failed to validate Elasticsearch versions for plan.

[ericywl]

Successfully reproduced locally as well. Redirected to elasticsearch team since I couldn't find anything else wrong in the logs.