MDEV-31811 deprecate old_mode values

mark non-default values of old_mode as deprecated.
print a warning when they're set from the command line and in SQL.

mysql-test/main/old-mode.test

@@ -2,11 +2,6 @@
 # Test 'old' mode
 #
 
-# Initialise
---disable_warnings
-drop table if exists t1,t2;
---enable_warnings
-
 create table t1 (a int, b varchar(200), c text not null) checksum=1;
 create table t2 (a int, b varchar(200), c text not null) checksum=0;
 insert t1 values (1, "aaa", "bbb"), (NULL, "", "ccccc"), (0, NULL, "");