Mark pageinspect's disk-accessing functions as parallel restricted.

These functions have been marked parallel safe, but the buildfarm's
response to commit e2933a6e1 exposed the flaw in that thinking:
if you try to use them on a temporary table, and they run inside
a parallel worker, they'll fail with "cannot access temporary tables
during a parallel operation".

Fix that by marking them parallel restricted instead.  Maybe someday
we'll have a better answer and can reverse this decision.

Back-patch to v15.  To go back further, we'd have to devise variant
versions of pre-1.10 pageinspect versions.  Given the lack of field
complaints, it doesn't seem worth the trouble.  We'll just deem
this case unsupported pre-v15.  (If anyone does complain, it might
be good enough to update the markings manually in their DBs.)

Discussion: https://postgr.es/m/E1ox94a-000EHu-VH@gemulon.postgresql.org

contrib/pageinspect/meson.build

@@ -33,6 +33,7 @@ install_data(
   'pageinspect--1.7--1.8.sql',
   'pageinspect--1.8--1.9.sql',
   'pageinspect--1.9--1.10.sql',
+  'pageinspect--1.10--1.11.sql',
   'pageinspect.control',
   kwargs: contrib_data_args,
 )