Dashboard: Migrate StoreStatsAndTopPerformersViewController.syncAllStats to use the Swift Concurrency model

[toupper]

As @jaclync suggested here #6890 (comment) it would be great to use the new Swift Concurrency model to rewrite that function. That would replace DispatchGroup and make the code more readable and lighter.

Hint:

We can run multiple tasks in parallel and wait for them as described here