Replace mirrors.tuna.tsinghua.edu.cn for China

The recent included app TLSFragment was censored on this mirror, this Chinese website returns HTTP 404 for its icon and screenshots (1, 2).

Considering the blocking of F-Droid.org by China since 2023, running an F-Droid mirror in China might be dangerous. I suggest removing any encouragement about this or even warning them about the risk.

Edit 1: This mirror was added in 2020 at mirror-monitor!9 (merged).