chore: update require-dev package name
chore: update require-dev package name

Running `composer install` gives the following warning
```
Deprecation warning: require-dev.mikey179/vfsStream is invalid, it should not contain uppercase characters. Please use mikey179/vfsstream instead. Make sure you fix this as Composer 2.0 will error.
```

GoogleCloudPlatform/appengine-php-sdk,GoogleCloudPlatform/appengine-php-sdk