Exclude package manifest tests from parallel tests

[Copilot]

The PHP 8.3 / Paratest 7.3 / PHPUnit 12.4 job failed when concurrent workers raced over the vendor symlink used by UsesVendor, leaving Carbon out of the generated manifest.

• Change: Mark PackageManifestTest as without-parallel, so it runs in the serial test jobs rather than concurrently.

#[UsesVendor]
#[Group('without-parallel')]
class PackageManifestTest extends TestCase

[coveralls]

coverage: 90.925%. remained the same — copilot/fix-github-actions-job into 11.x