Bump minimum version of Bison to 2.3

Bump minimum version of Bison to 2.3

Since the retirement of some older buildfarm members, the oldest Bison
that gets regular testing is 2.3. MacOS ships that version, and will
continue doing so for the forseeable future because of Apple's policy
regarding GPLv3. While Mac users could use a package manager to install
a newer version, there is no compelling reason to force them do so at
this time.

Reviewed by Andres Freund
Discussion: https://www.postgresql.org/message-id/1097762.1662145681@sss.pgh.pa.us