docs: Add deprecation notice to README files directing users to X6

[Copilot]

XFlow will no longer be actively maintained. Users should migrate to X6, which will continue to receive updates.

Changes

• README.md: Added deprecation notice in English at the top of the file
• README.zh-Hans.md: Added deprecation notice in Chinese (简体中文) at the top of the file

Both notices are prominently placed after the language selector, include a warning emoji (⚠️), and link directly to the X6 repository.

Original prompt

在 README 中加入描述，表达：考虑到用户量情况，未来不会继续迭代 XFlow，相关需求强烈建议使用 X6，X6 是会持续维护的。

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[changeset-bot]

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Latest commit: 23751c4

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