Move MongoDB config under "mongodb" subkey

[glensc]

New mongodb config keys:

    'mongodb' => [
        // 'hostname' and 'port' are used to build DSN for MongoClient
        'hostname' => getenv('XHGUI_MONGO_HOSTNAME') ?: '127.0.0.1',
        'port' => getenv('XHGUI_MONGO_PORT') ?: 27017,
        // The database name
        'database' => getenv('XHGUI_MONGO_DATABASE') ?: 'xhprof',
        // Additional options for the MongoClient constructor,
        // for example 'username', 'password', or 'replicaSet'.
        // See <https://www.php.net/mongoclient_construct#options>.
        'options' => [],
        // An array of options for the MongoDB driver.
        // Options include setting connection context options for SSL or logging callbacks.
        // See <https://www.php.net/mongoclient_construct#options>.
        'driverOptions' => [],
    ],

Support for previous keys remains for undefined future.

Fixes #267