Clarify that we specify the client, not agent version

Why is this change being made?

Hotfix to our docs.

The server version is whatever is deployed to the environment we're running against. Most specifically, these tests only run against staging ang staging-canary, so the server version is whatever is currently deployed there. What we define in this file is the version of the client (agentk) we're using to connect to it.

By the way, we should make this also dynamic, so that we take the latest released version, or better, the version that is deployed in the environment we're running against. But this MR simply clarifies the current behaviour for now.

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