Fix the volatility setting of json{b}_strip_nulls

Fix the volatility setting of json{b}_strip_nulls

Commit 4603903d294 unfortunately reset the volatility of these functions
to STABLE whereas they had previously been set to IMMUTABLE. We can't
force a catalog update in the stable release, although a pg_update would
probably do the trick. A simpler fix, though, for affected users is
probably a simple catalog surgery along the lines of:

   UPDATE pg_proc SET provolatile = 'i' WHERE oid in (3261,3262);

Applied to 18 only. In master we are planning to get rid of the separate
redeclarations for defaults in system_functions.sql.

Bug: #19409
Reported-By: Lucio Chiessi <lucio.chiessi@trustly.com>
Discussion: https://postgr.es/m/19409-e16cd2605e59a4af@postgresql.org