Open Buy Minutes in new window if targeting CDot

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR fixes a regression introduced with !68009 (merged). When opening the purchase flow in CustomersDot, the page should still open in a new window/tab.

Screenshots or screen recordings

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before	after

How to set up and validate locally

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

Example below:

1. Make sure to have set GITLAB_LICENSE_MODE=test on gdk (see here)
2. Make sure to have a Group with a purchased plan
3. Visit /groups/<your-group-name>/-/usage_quotas#pipelines-quota-tab

• With :new_route_ci_minutes_purchase enabled, the target should be _self
• With :new_route_ci_minutes_purchase disabled, the target should be _blank

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.