Do not hardcode PG_PROTOCOL_LATEST in NegotiateProtocolVersion

Do not hardcode PG_PROTOCOL_LATEST in NegotiateProtocolVersion

We shouldn't ask the client to use a protocol version later than the
one that they requested. To avoid that, if the client requests a
version newer than the latest one we support, set FrontendProtocol
to the latest version we support, not the requested version. Then,
use that value when building the NegotiateProtocolVersion message.
(It seems good on general principle to avoid setting FrontendProtocol
to a version we don't support, anyway.)

None of this really matters right now, because we only support a
single protocol version, but if that ever changes, we'll need this.

Jelte Fennema-Nio, reviewed by me and incorporating some of my
proposed wording

Discussion: https://postgr.es/m/CAGECzQTyXDNtMXdq2L-Wp=OvOCPa07r6+U_MGb==h90MrfT+fQ@mail.gmail.com