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_bug14007649.result

@@ -17,6 +17,7 @@ insert into `t1` (rowid, f1, f2) values (1, 1, 10), (2, 1, NULL);
 connect  a,localhost,root,,;
 connect  b,localhost,root,,;
 connection a;
+set innodb_snapshot_isolation=OFF;
 start transaction with consistent snapshot;
 connection b;
 start transaction;