Remove hardcode of "gitlab.com" in spec

What does this MR do and why?

Related Jihu issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3191

There are three hardcodes about gitlab.com in the spec, which caused the test error in Jihu:

• https://jihulab.com/gitlab-cn/gitlab/-/jobs/8299391
• https://jihulab.com/gitlab-cn/gitlab/-/jobs/8299386
• https://jihulab.com/gitlab-cn/gitlab/-/jobs/8299380

Failure/Error: expect(flash[:alert]).to eq(error_message)
   expected: "Your account has been blocked. Contact https://support.gitlab.com for assistance."
        got: "Your account has been blocked. Contact https://support.gitlab.cn for assistance."
   (compared using ==)

The current MR changes these hardcodes to dynamically concatenated strings.

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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cc @mtan-gitlab @icbd