Extracting cross-version-upgrade knowledge from buildfarm client

This is a followup to the discussion at [1], in which we agreed that
it's time to fix the buildfarm client so that knowledge about
cross-version discrepancies in pg_dump output can be moved into
the community git tree, making it feasible for people other than
Andrew to fix problems when we change things of that sort.
The idea is to create helper files that live in the git tree and
are used by the BF client to perform the activities that are likely
to need tweaking.

Attached are two patches, one for PG git and one for the buildfarm
client, that create a working POC for this approach. I've only
carried this as far as making a helper file for HEAD, but I believe
that helper files for the back branches would mostly just need to
be cut-down versions of this one. I've tested it successfully with
cross-version upgrade tests down to 9.3. (9.2 would need some more
work, and I'm not sure if it's worth the trouble --- are we going to
retire 9.2 soon?)

I'm a very mediocre Perl programmer, so I'm sure there are stylistic
and other problems, but I'm encouraged that this seems feasible.

Also, I wonder if we can't get rid of
src/bin/pg_upgrade/upgrade_adapt.sql in favor of using this code.
I tried to write adjust_database_contents() in such a way that it
could be pointed at a database by some other Perl code that's
not the buildfarm client.

regards, tom lane

[1] https://www.postgresql.org/message-id/951602.1673535249%40sss.pgh.pa.us