pgsql: doc: mention pg_upgrade extension script - Mailing list pgsql-committers

doc:  mention  pg_upgrade extension script

Since commit e462856a7a, pg_upgrade automatically creates a script to
update extensions, so mention that instead of ALTER EXTENSION.

Backpatch-through: 9.6

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ff18b8d1b169b2bf47daab7d92c5376bc19c0748

Modified Files
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doc/src/sgml/ref/pgupgrade.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)