Deprecate fallback connection to determine platform

[morozov]

The fallback connection only allows detection of the server version but doesn't allow anything else. E.g. it cannot be used to create the database used in the configuration that doesn't yet exist (which one would think would be the intended use case).

$connection = DriverManager::getConnection([
    'driver' => 'mysqli',
    'host'   => '127.0.0.1',
    'user'   => 'root',
    'dbname' => 'non_existing_database',
]);

echo get_class($connection->getDatabasePlatform()), PHP_EOL;
// Doctrine\DBAL\Platforms\MySQL80Platform

try {
    $connection->executeStatement('CREATE DATABASE non_existing_database');
} catch (Exception $e) {
    echo $e->getMessage(), PHP_EOL;
    // An exception occurred in the driver: Unknown database 'non_existing_database'
}

If the configured database doesn't exist, in order to create it, the user will have to use an admin connection anyway (like the test suite does). The feature seems to have worked like the above since its implementation (#2671).

Besides having questionable value and not being fully portable and not having integration tests, this code has required quite some maintenance, most recently in #4764 (comment).

[greg0ire]

I was wondering what this change would affect and I thought the command doctrine:database:create from the Symfony bundle would be it, but it appears to already be doing what you recommend here (by unsetting dbname): https://github.com/doctrine/DoctrineBundle/blob/1af518a2fd74fdf2d8b78de1a99899e44e8db8b9/Command/CreateDatabaseDoctrineCommand.php#L95-L97