Bump minimum Perl version to 5.14

Bump minimum Perl version to 5.14

The oldest vendor-shipped Perl in the buildfarm is 5.14.2, which is
the last version that Debian Wheezy shipped. That OS is EOL, but we
keep it running because there is no other convenient way to test certain
non-mainstream 32-bit platforms. There is no bugfix in the 5.14.2 release
that is required, and yet it's also not the latest minor release --
that would be 5.14.4. To clarify the situation, we have thus arranged the
buildfarm to test 5.14.0. That allows configure scripts and documentation
to state 5.14 without fine print.

The MSVC build didn't check the version, since our previous minimum 5.8.3
was considered too old to check for on Windows. We will need a check for
Windows sometime during the v16 cycle, but that could be rendered moot
by the impending Meson conversion, so it seems safe to just document
the requirement for now.

Reviewed by Tom Lane
Discussion: https://www.postgresql.org/message-id/20220902181553[email protected]