Doc: fix outdated protocol version.

Doc: fix outdated protocol version.

In the description of StartupMessage, the protocol version was left
3.0. Instead of just updating it, this commit removes the hard coded
protocol version and shows the numbers as an example. This makes that
the part of the doc does not need to be updated when the version is
changed in the future.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/20250626.155608.568829483879866256.ishii%40postgresql.org