Documentation is outdated: IPROTO_VERSION is at least 6, not 3

Bug description

Hi Tarantool team,

I'm writing to report inaccuracy in the official documentation regarding the iproto protocol version. I have found in the source code that the documentation needs to be updated.

The documentation for IPROTO_VERSION states that the latest protocol version is 3. https://www.tarantool.io/en/doc/latest/reference/internals/iproto/keys/#iproto-version

However, evidence from both live traffic analysis and the official Python connector's source code indicates that the actual protocol version in use is significantly higher. CONNECTOR_IPROTO_VERSION = 6
https://github.com/tarantool/tarantool-python/blob/master/tarantool/connection.py#L885

Please update the documentation to reflect the current state of the iproto protocol. otherwise, please explain how the above should be interpreted

Thank you for your attention to this matter.

Start with the IPROTO_VERSION documentation page linked in the issue and compare its stated version with tarantool-python's connection.py around line 885. Confirm whether the documented value should be updated to 6 or requires an explanation of the version distinction, then update the page and verify the rendered wording.