Improve Flutter description

[jjangga0214]

1. Whether an app is native matters. Flutter is to build native app, so I added the word "native".
2. Whether to write code once, or twice is a significant factor as well. So I explicitly mentioned "single codebase".
3. Which language to use counts much. Hence I let people see "Dart".

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Please read it multiple times. I spent a lot of time on these guidelines and most people miss a lot.

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I REMOVED THE SECOND SECTION OF PR TEMPLATE FOR SAVING SCROLLING HEIGHT AS THIS PR IS FOR UPDATE