This is a placeholder ticket. The work for mixed version cluster scenario testing as it relates to extensions optimizations should be done by the extensions search team on Atlas. As per the design:

The most interesting case to test will be a mixed version cluster scenario. Specifically, we will have the case where the router/mongos is going to be doing one pass of optimizations on the old extension, and then they will serialize themselves on that older version, and have the shards digest that output to parse from. 

To test this, we must have old and new versions of the real $vectorSearch and $rerank extensions. We will upgrade server binaries and extensions together on a cluster from 8.3 → 9.0 while running queries against it and ensure that the upgrade is seamless.

This test case would not be feasible to commit and run in Evergreen. It is also largely concerned with the upgrade process for the extension, rather than the server.