Deprecate Driver::getSchemaManager() in favor of AbstractPlatform::createSchemaManager()

[morozov]

Q	A
Type	improvement, deprecation

The operation of creating a schema manager doesn't belong to the driver layer: the driver represents transport. Different transports use the same schema management logic for the same platform. The very fact that Driver::getSchemaManager() is implemented only in abstract driver classes illustrates that.

At the same time, the schema management logic may depend on the platform vendor and the platform version:

dbal/src/Schema/MySQLSchemaManager.php

Lines 244 to 248 in d4d9389

	if ($this->_platform instanceof MariaDb1027Platform) {
	$columnDefault = $this->getMariaDb1027ColumnDefault($this->_platform, $tableColumn['default']);
	} else {
	$columnDefault = $tableColumn['default'];
	}

But currently, it's challenging to have different schema managers for different platform vendors or versions.