This was overlooked in commit 473b93287, which introduced DROP ACCESS
METHOD. Although that command is restricted to superusers, we don't want
even superusers dropping the built-in methods; "DROP ACCESS METHOD btree"
in particular is unrecoverable from. Pin these objects in the same way
that other initdb-created objects are pinned.
I chose to bump catversion for this fix. That's not absolutely necessary
perhaps, but it will ensure that no 9.6 production systems are missing
the pin entries.
16ea51a263