pg_upgrade supported versions policy

Hi,

I feel like we ought to trim the support for a few old versions from
pg_upgrade.  In my particular case I don't really think it's reasonable
to test < 9.0 versions for pg_largeobject_metadata migrations. But I
think we should create a policy that's the default, leaving individual
cases aside.

I can see a few possible policies:

1) Support upgrading from the set of releases that were supported when
   the pg_upgrade target version was released. While some will argue that
   this is fairly short, people above it can still upgrade ~10 years
   worth of versions with a single intermediary upgrade.
2) Same, but +2 releases or such.
3) Support until it gets too uncomfortable.
4) Support all versions remotely feasible (i.e. don't drop versions that
   still can be compiled)

I personally think 1 is preferrable and 2 is acceptable.

Greetings,

Andres Freund