Remove portability hazard in unsafe_tests/sql/guc_privs.sql.

Remove portability hazard in unsafe_tests/sql/guc_privs.sql.

This new-in-v15 test case assumed it could set max_stack_depth as high
as 2MB.  You might think that'd be true on any modern platform but
you'd be wrong, as I found out while experimenting with NetBSD/hppa.

This test is about privileges not platform capabilities, so there seems
no need to use any value greater than the 100kB setting already used
in a couple of places in the core regression tests.  There's certainly
no call to expect people to raise their platform's default ulimit just
to run this test.