libpq: Bump protocol version to version 3.2 at least until the first/second beta

The main reason that libpq does not request protocol version 3.2 by
default is because other proxy/server implementations don't implement
the negotiation. This is a bit of a chicken and egg problem: We don't
bump the default version that libpq asks, but proxies will only
implement version negotation when their users run into issues. So I'm
proposing that we bump the default protocol version that libpq requests
on the master branch, so that users running the latest master against a
proxy that does not support the version negototian will be notified.
They can then push the author of the proxy to implement the
NegototiateProtocolVersion message. 

Depending on how this works in practice we'll likely still want to
revert this change before we actually release PG19. If we do that before
19beta1 we still have roughly half a year where people will test the
ecosystem. I'd even suggest that we only revert before 19beta2, so that
people testing beta1 will also be testing the ecosystem for version
negotiation issues. In any case, the sooner we commit this the more
testing we get, 

Note that users still have a way out to connect to the server by
manually setting max_protocol_version=3.0 in the connection string.