refactor(components): [dialog] using addUnit in the use-dialog

[ryuhangyeong]

Description

hello. We're looking at the components little by little, looking for bugs or refactoring deprecated APIs and code.

I think you can achieve what you want a little more concisely using the addUnit function.

thank you Element Plus Team!

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Hello @ryuhangyeong, thank you for contributing to element-plus, please see our guideline to see how to make contribution

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🧪 Playground Preview: https://element-plus.run/?pr=8304
Please comment the example via this playground if needed.