Update to budget limits in Appwrite Cloud

We've updated how budget limits work in Appwrite Cloud to provide more control over organization scaling.

What's changed

• Previously, setting the budget to 0 meant the budget was disabled (unlimited usage), and null was not supported.
• Now, null explicitly disables the budget, while 0 is a valid budget limit, meaning the organization will not scale beyond its allocated resources.

This change ensures clearer budget management and better control over scaling limits.