feat(db): set Doctrine to use nest transactions with savepoints

Using nested transactions without savepoints is actually deprecated by Doctrine:
https://www.doctrine-project.org/projects/doctrine-dbal/en/current/reference/transactions.html#transaction-nesting

Without savepoints, a nested transaction can be rollbacked but not
handled properly in the "real" transaction, leading to the following
error:
Transaction commit failed because the transaction has been marked for rollback only.

Ref
#36528 (comment)
(and possibly) #38902 (comment)

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/private/DB/Connection.php

@@ -127,6 +127,8 @@ public function __construct(
 			$this->dbDataCollector->setDebugStack($debugStack);
 			$this->_config->setSQLLogger($debugStack);
 		}
+
+		$this->setNestTransactionsWithSavepoints(true);
 	}
 
 	/**