在上游镜像站更新 PkgServerClient 的使用

[johnnychen94]

因为镜像站切换的功能现在放在 PkgServerClient 下去做了，所以一些帮助文档需要做相应的更新

目前我已知的镜像站有：

• [ ] BFSU/TUNA https://github.com/tuna/mirror-web/blob/master/help/_posts/2020-05-25-julia.md
• [ ] SJTUG https://github.com/sjtug/portal/blob/master/content/post/mirror-help/julia.md
• [ ] USTC https://github.com/ustclug/mirrorhelp/blob/master/source/julia.rst
• [ ] SUSTech https://github.com/SUSTC/mirrors-web-cra/blob/98b7ddc6bbd8d23079b7125b3967362d415e1040/_posts/help/2020-05-25-julia.md

以及

• [x] discourse https://discourse.juliacn.com/t/topic/2969

cc: @ Ionizing