Backport of 'Fix broken trial CTAs on SM GitLab Credits dashboard' into 19-0-stable-ee

What does this MR do and why?

Backport of !236883 (merged) into 19-0-stable-ee.

On Self-Managed EE instances with a trial license, the admin/gitlab_credits_dashboard page renders FreeTierTrialApp which mounts two cards whose CTAs (Purchase credits, Upgrade to Premium) injected paths that the admin haml never passed — both buttons rendered with empty/undefined href.

• Adds upgrade_button_path and purchase_credits_path data attributes to ee/app/views/admin/gitlab_credits_dashboard/index.html.haml, wired to the promo URL and subscription_portal_self_managed_purchase_credits_url.
• Renames the injection key from gitlab_com_purchase_credits_path / gitlabComPurchaseCreditsPath to purchase_credits_path / purchaseCreditsPath in the usage_quotas/usage_billing scope. The attribute now serves both SaaS (gitlab.com URL) and SM (self-managed URL); the old name was misleading. The unrelated ai/settings/ usage is left untouched since it remains SaaS-only.

Cherry-picked from commit 5be95056.

References

• Default-branch MR: !236883 (merged)
• Resolves #600173
• Related UAT: gitlab-org/gitlab#598134

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

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