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.

src/test/modules/unsafe_tests/expected/guc_privs.out

@@ -436,9 +436,9 @@ ERROR:  parameter "autovacuum_work_mem" cannot be changed now
 ALTER SYSTEM RESET temp_file_limit;  -- ok
 SET TimeZone = 'Europe/Helsinki';  -- ok
 RESET TimeZone;  -- ok
-SET max_stack_depth = 2048;  -- ok, privileges have been granted
+SET max_stack_depth = '100kB';  -- ok, privileges have been granted
 RESET max_stack_depth;  -- ok, privileges have been granted
-ALTER SYSTEM SET max_stack_depth = 2048;  -- ok, privileges have been granted
+ALTER SYSTEM SET max_stack_depth = '100kB';  -- ok, privileges have been granted
 ALTER SYSTEM RESET max_stack_depth;  -- ok, privileges have been granted
 SET lc_messages = 'C';  -- fail, insufficient privileges
 ERROR:  permission denied to set parameter "lc_messages"

src/test/modules/unsafe_tests/sql/guc_privs.sql

@@ -176,9 +176,9 @@ SET autovacuum_work_mem = 50;  -- cannot be changed now
 ALTER SYSTEM RESET temp_file_limit;  -- ok
 SET TimeZone = 'Europe/Helsinki';  -- ok
 RESET TimeZone;  -- ok
-SET max_stack_depth = 2048;  -- ok, privileges have been granted
+SET max_stack_depth = '100kB';  -- ok, privileges have been granted
 RESET max_stack_depth;  -- ok, privileges have been granted
-ALTER SYSTEM SET max_stack_depth = 2048;  -- ok, privileges have been granted
+ALTER SYSTEM SET max_stack_depth = '100kB';  -- ok, privileges have been granted
 ALTER SYSTEM RESET max_stack_depth;  -- ok, privileges have been granted
 SET lc_messages = 'C';  -- fail, insufficient privileges
 RESET lc_messages;  -- fail, insufficient privileges