Fix default signature length for gist_ltree_ops

Fix default signature length for gist_ltree_ops

911e702077 implemented operator class parameters including the signature length
in ltree.  Previously, the signature length for gist_ltree_ops was 8.  Because
of bug 911e702077 the default signature length for gist_ltree_ops became 28 for
ltree 1.1 (where options method is NOT provided) and 8 for ltree 1.2 (where
options method is provided).  This commit changes the default signature length
for ltree 1.1 to 8.

Existing gist_ltree_ops indexes might be corrupted in various scenarios.
Thus, we have to recommend reindexing all the gist_ltree_ops indexes after
the upgrade.

Reported-by: Victor Yegorov
Reviewed-by: Tomas Vondra, Tom Lane, Andres Freund, Nikita Glukhov
Reviewed-by: Andrew Dunstan
Author: Tomas Vondra, Alexander Korotkov
Discussion: https://postgr.es/m/17406-71e02820ae79bb40%40postgresql.org
Discussion: https://postgr.es/m/d80e0a55-6c3e-5b26-53e3-3c4f973f737c%40enterprisedb.com