pgsql: Fix breakage in AlterFunction().

Fix breakage in AlterFunction().

An ALTER FUNCTION command that tried to update both the function's
proparallel property and its proconfig list failed to do the former,
because it stored the new proparallel value into a tuple that was
no longer the interesting one. Carelessness in 7aea8e4f2.

(I did not bother with a regression test, because the only likely
future breakage would be for someone to ignore the comment I added
and add some other field update after the heap_modify_tuple step.
A test using existing function properties could not catch that.)

Per report from Bryn Llewellyn. Back-patch to all supported branches.

Discussion: https://postgr.es/m/8AC9A37F-99BD-446F-A2F7-B89AD0022774@yugabyte.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9130f8cbb91954f7a40de70c014c01b552df31da

Modified Files
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src/backend/commands/functioncmds.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)