Update some links in the Billing page

1. For the group which is using the free plan, in its Settings -> Billing page, we need to

   • Change the link of frequently asked questions from https://about.gitlab.com/gitlab-com/#faq to https://about.gitlab.cn/gitlab-cn/#faq. But it seems we do not have the latter in our website yet, so we may need @zhendi to help.
   • Change the link of See all Free Plan features, See all 高级版 features and See all 旗舰版 features from https://about.gitlab.com/pricing/gitlab-com/feature-comparison to https://about.gitlab.cn/pricing/gitlab-cn/feature-comparison. But it seems we do not have the latter in our website yet, what we have now is https://about.gitlab.cn/pricing/gitlab-com/feature-comparison which seems not correct to me (we should change gitlab-com to gitlab-cn), so we may need @zhendi to help as well.

2. For the group which is using a paid plan (e.g. ultimate plan), in its Settings -> Billing page, we need to

   • Change the link of Pricing page from https://about.gitlab.com/pricing to https://about.gitlab.cn/pricing.