fix: cashApp not destroyed on unmount

[testerez]

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Description

On our checkout page, we have a back button that will unmount the CashAppPay component. When we try to pay again, we get this error:

Initializing Cash App Pay failed CashAppPayError: already rendered. must destroy before rendering again

This PR is calling cashApp?.destroy(); on useEffect cleanup function so that it's properly destroyed on unmount and works as expected when re-mounting.