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/main/partition_innodb_semi_consistent.test

@@ -29,6 +29,7 @@ set autocommit=0;
 select * from t1 where a=3 lock in share mode;
 connection b;
 set binlog_format=mixed;
+set innodb_snapshot_isolation=OFF;
 set session transaction isolation level repeatable read;
 set autocommit=0;
 -- error ER_LOCK_WAIT_TIMEOUT