MDEV-35124 Set innodb_snapshot_isolation=ON by default

From the very beginning, the default InnoDB transaction isolation level
REPEATABLE READ does not correspond to any well formed definition.
The main issue is the lack of write/write conflict detection.
To fix that and to make REPEATABLE READ correspond to Snapshot Isolation,
b8a6719889 introduced the Boolean
session variable innodb_snapshot_isolation. It was disabled by default
in order not to break any user applications.

In a new major version of MariaDB Server, we had better enable this
parameter by default.

mysql-test/suite/innodb/r/innodb-semi-consistent.result

@@ -20,6 +20,7 @@ commit;
 connection a;
 commit;
 connection b;
+set innodb_snapshot_isolation=off;
 set session transaction isolation level read committed;
 update t1 set a=10 where a=5;
 connection a;