Remove references to `mirror.sjtu.edu.cn` from `README.md`

[linghengqian]

• https://mirror.sjtu.edu.cn/flutter_infra cannot actually be used as the FLUTTER_STORAGE_BASE_URL environment variable; this is an error introduced by Update outdated Flutter SDK mirror sites #10 . 🤣 That PR was primarily intended to remove the defunct Tsinghua mirror.
• Given that mirror.sjtu.edu.cn has announced (at https://mirror.sjtu.edu.cn/news#2026-08-23-mirrorz ) its participation in the https://mirrors.cernet.edu.cn/ , the following entry:

SJTU (Shanghai Jiao Tong University): https://mirror.sjtu.edu.cn/flutter_infra

should be replaced with:

CERNET (China Education and Research Network): https://mirrors.cernet.edu.cn/flutter

• Also see https://mp.weixin.qq.com/s/ipei4VrTTII0DgZqzMIzyw .
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