Fix spec of preferred_language_switcher.rb

What does this MR do and why?

Fix failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/17353862

错误原因：Upstream spec 中使用了 about.gitlab.com 这样的域名，在极狐这边用正则匹配失败（只能匹配 cn 域名）。

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