Set default transaction isolation level to READ-COMMITTED in MySQL/MariaDB

Outside of fixing this, I'm not sure how D7 would be OK with this change.
D7 has no requirement for this variable, like D10+ has, though it is recommended in several guides

dimitriskr → changed the visibility of the branch 3405511-set-default-transaction-read-committed to hidden.

dimitriskr → changed the visibility of the branch 3405511-set-default-transaction-read-committed to active.

We probably need to test this on D7 before making this change to production.