MemoryStoreService:UpdateAsync types are wrong #1394

The transform function is supposed to accept the old value and sort key, and return both the new value and sort key, or nil if the update is supposed to be cancelled. The current implementation does not have the sort key in the input or return, and also the return is not nullable when it should be.

Additionally the return value of the method is supposed to reflect the return of the transform function, which includes the sort key, and should also be nullable.

https://create.roblox.com/docs/reference/engine/classes/MemoryStoreSortedMap#UpdateAsync