Fixes required for cross version update testing

Over at [1] there was some discussion of moving knowledge of what's
required to be fixed from old branch repos to be able to upgrade them
into the core code, instead of having it reside in a buildfarm client
module.

Here's a piece of WIP for that, in the form of a perl module that
provides a function that takes an old version number / tag and provides
the set of sql statements that need to be run to make the old repo
upgradeable. It still needs a good deal of polish, but it's a start.

The advantage is that it makes it far less likely that the buildfarm
maintainer (i.e. me for now) is a bottleneck in fixing issues that arise
from development. This is by far the biggest area where we have seen
buildfarm breakage for cross version upgrade testing.

cheers

andrew

[1]  https://postgr.es/m/951602.1673535249@sss.pgh.pa.us

--
Andrew Dunstan
EDB: https://www.enterprisedb.com