pgsql: In pg_upgrade, disallow migration of 8.3 clusters using contrib/

In pg_upgrade, disallow migration of 8.3 clusters using contrib/ltree
because its internal format was changed in 8.4.

Backpatch to 9.0 and 9.1.

Report by depesz, diagnosis by Tom.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3ced32d24e14af1877171c085a635b4ccdf8930c

Modified Files
--------------
contrib/pg_upgrade/check.c           |    1 +
contrib/pg_upgrade/pg_upgrade.h      |    1 +
contrib/pg_upgrade/version_old_8_3.c |   81 ++++++++++++++++++++++++++++++++++
doc/src/sgml/pgupgrade.sgml          |    5 ++
4 files changed, 88 insertions(+), 0 deletions(-)