pgsql default version

[Taylor R Campbell <campbell+pkgsrc-users%mumble.net@localhost>]

The default pgsql version has been 9.5 since 2016.  9.4 was just
eol'd, 9.5 will be eol'd in under a year, and the most recent release
is 12 now.

Although we have multi-version packages like postgresql*-client, not
all pgsql-dependent packages have the pgsql version embedded in the
pkgname, e.g. databases/py-postgresql -- and different versions of
postgresql*-client conflict with one another.

(Perhaps everything that depends on pgsql _should_ be multi-versioned
-- but that's a more ambitious discussion for a more ambitious day.)

Should we bump PGSQL_VERSION_DEFAULT?

At the time it was bumped to 9.5, 9.5 was only a few months old and
9.6 was a few weeks old.  Now 9.5 is several years old and 12 is
several months old.