Make PostgreSQL::Test::Cluster compatible with all live branches

Make PostgreSQL::Test::Cluster compatible with all live branches

We do this via a subclass for any branch older than the minimum known
to be compatible with the main package (currently release 12).

This should be useful for constructing cross-version tests.

In theory this could be extended back any number of versions, with
varying degrees of compatibility.

Reviewed by Michael Paquier and Dagfinn Ilmari Mannsåker

Discussion: https://postgr.es/m/a3efd19a-d5c9-fdf2-6094-4cde056a2708@dunslane.net