psql: Report full protocol version in \conninfo output.

psql: Report full protocol version in \conninfo output.

Commit bba2fbc6238 modified \conninfo to display the protocol version
used by the current connection, but it only showed the major version (e.g., 3).

This commit updates \conninfo to display the full protocol version (e.g., 3.2).
Since support for new version 3.2 was added in v18, and the server supports
both 3.0 and 3.2, showing the complete version helps users understand
exactly which protocol version the current session is using.

Although this is a minor behavior change, it's considered a fix for
an oversight in the original patch and is included in v18.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: http://postgr.es/m/685961b8-b6ce-40bb-b2d5-c2ff135d3388@oss.nttdata.com