change the way that merge state within useOptimistic demo

[felismargarita]

This code change aims to help developers better understand the usage of useOptimistic that it's important to set a correct merge state logic to avoid incorrect mid-state rendering.

Here is a screenshot that the original code does not handle the merge logic properly that makes a same message displayed twice:

#7379

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Name	Status	Preview	Comments	Updated (UTC)
19-react-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 19, 2024 5:57am
react-dev	✅ Ready (Inspect)	Visit Preview		Dec 19, 2024 5:57am

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[karthik-awebon]

Hello @felismargarita the existing code effectively demonstrates the use of the UseOptimistic Hook with a simple example. Adding unnecessary complexity to handle duplicate messages detracts from the core purpose of the demo.

Additionally, your PR does not function as intended. The use of time as a unique identifier for messages is flawed, as identical messages can still appear multiple times if their timestamps differ, as evidenced in the screenshot.