[UI v2] feat: simplfies invalidating all variable queries

[devinvillarosa]

What does this PR do and why?

Simplifies invalidating all queries for ['variables'].

Looking at the code, it looks like we want to invalidate all queries starting with 'variables' as the key.
The simplest way of doing it is just passing queryKey: variableKeys.all, or queryKey: ['variables']. You don't need to iterate through all queries and evaluate each one for this case

React Query's query key matching is simple. It will match query keys on how specific the key sequence is being passed.
eg:
Passing['variables'], will invalidate all query keys starting with 'variables'.
Passing ['variables', 'filtered-count], will only invalidate all queries that matches up to this sequence, such as ['variables', 'filter-count', 'foo'] ['variables', 'filter-count', 'foo', 'bar'] ['variables', 'filter-count', 'baz', {id: 1}]

🗒️ Sidenote and future enhancement: this is why query factories is a useful model. You can structure which queries you want to invalidate, and invalidate certain sequences. This is why all() is mapped with ['variables']. If you want to invalidate all queries relating to a 'variables', just pass queryKey: ['variables']
(📖 https://tkdodo.eu/blog/the-query-options-api#query-factories)

Docs:
https://tanstack.com/query/latest/docs/framework/react/guides/query-invalidation#query-matching-with-invalidatequeries

Future
We should centralize the query keys so that they're accessible in the mutations called in variables/data-table/cells. Additionally, these mutations should probably be re-usable as hooks
https://github.com/PrefectHQ/prefect/blob/main/ui-v2/src/components/variables/data-table/cells.tsx#L40-L47

Checklist

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Related to #15512

[desertaxle]

Cool! I agree that the delete variable mutation should be moved into ui-v2/src/hooks/variables.ts. I must've missed that when I was reorganizing things. I can submit a PR to move it!