Replace experimental_use_rmcp_client with [features].rmcp_client

What is the type of issue?

Documentation is confusing

What is the issue?

In the documentation and the CLI there is still a mention of experimental_use_rmcp_client.
While it still works, it is deprecated and should refer to [features].rmcp_client

Where did you find it?

• Documentation: https://github.com/openai/codex/blob/b5dd1890671d58dd92c29dc4ffa6432ecb48d54c/docs/config.md?plain=1#L474)
• CLI: https://github.com/openai/codex/blob/b5dd1890671d58dd92c29dc4ffa6432ecb48d54c/codex-rs/cli/src/mcp_cmd.rs#L287