fix: revert JSON serialization workaround, upgrade SDK to 6.3.1 instead

v2.0.1's useJson() workaround was wrong: the JSON request serializer
silently drops nested single-object fields (e.g. CustomerRef, which
QuickBooks requires), and the JSON response deserializer never returns
a typed entity, so $result->Id is always null downstream. That breaks
every dsCall() site, worse than the bug it was meant to fix.

The actual root cause was that quickbooks/v3-php-sdk 5.4.7 predates
QuickBooks Online's new invoice AllowOnlinePayPalPayment field, so its
XML deserializer throws on responses that include it. SDK 6.3.1
declares the field, so upgrading resolves the original error while
keeping the fully-supported XML path (and closes out the SDK bump
Dependabot already had queued in #3).

Verified: deserializing a response containing AllowOnlinePayPalPayment
throws RuntimeException on 5.4.7 and returns a correctly typed
IPPInvoice (with Id populated) on 6.3.1.

tests/ClientTest.php

@@ -1,6 +1,5 @@
 <?php
 
-use QuickBooksOnline\API\Core\Http\Serialization\SerializationFormat;
 use QuickBooksOnline\API\DataService\DataService;
 use QuickBooksOnline\API\Exception\ServiceException;
 use Wearepixel\QuickBooks\Client;
@@ -131,27 +130,6 @@
 it('returns a report service using the data service')
     ->skip('Once we figure out how to test around the static DataService::Configure');
 
-it('configures the data service to use JSON serialization so unrecognised QuickBooks response fields do not throw', function () {
-    $this->token_mock
-        ->shouldReceive('getAttribute')
-        ->twice()
-        ->with('hasValidAccessToken')
-        ->andReturnFalse();
-
-    $this->token_mock
-        ->shouldReceive('getAttribute')
-        ->once()
-        ->with('hasValidRefreshToken')
-        ->andReturnFalse();
-
-    $format = $this->client
-        ->getDataService()
-        ->getServiceContext()
-        ->IppConfiguration->Message->Request->SerializationFormat;
-
-    expect($format)->toBe(SerializationFormat::Json);
-});
-
 it('has logging off by default')
     ->skip('Have to figure out how to test this with the new code in 5.x');