Re-add unnecessary version checks to pg_dump, to reduce diff vs. upstream.

GPDB's pg_dump only supports dumping from server versions based on
PostgreSQL 8.2 and above, while PostgreSQL's pg_dump supports much older
versions. In GPDB's version, many of the version checks to deal with
older versions had been removed, but that caused a lot of diffs against
upstream, because of changed indentation.

To reduce our diff footprint, put back those versions checks. But actually
trying to deal with very old server versions seems futile, so instead of
resurrecting all the code, just put an error message into the branches
where upstream e.g. constructs queries that work with older versions. The
version checks should never actually fail, because we check the server
version once at the beginning of pg_dump. But it's nice to have something
in those branches, to document the fact that there was more code there in
the upstream, and to keep the formatting the same as in upstream.