Update Duo Pro usage button to Purchase seats

What does this MR do and why?

The purpose of this MR is to update the Add seats button on the Duo Seat Utilization page to be Purchase seats with an external link icon.

Related Issues:

• #507331 (closed) - Resolve discrepancy between CTA label on Duo settings homepage and seat assignment page (Duo Pro only)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After	Screen Recording
		Screen_Recording_2025-02-11_at_6.32.30_PM

How to set up and validate locally

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

For frontend changes:

You will need an GitLab Ultimate license for these changes as well as a connection to CustomersDot. Also, make sure you have Duo set up with a group as indicated here.

Make sure you have Duo is set up for a group, as noted here.

In gitlab-development-kit

• env.runit example

export GITLAB_SIMULATE_SAAS=1
export GITLAB_LICENSE_MODE=test
export LLM_DEBUG=1
export AI_GATEWAY_URL=http://0.0.0.0:5052
export CLOUD_CONNECTOR_SELF_SIGN_TOKENS=1
export CUSTOMER_PORTAL_URL=https://customers.staging.gitlab.com/

• gdk.yml example to set up CDot

license:
  customer_portal_url: https://customers.staging.gitlab.com
  license_mode: test

Testing the Group General Settings Page

Group - SaaS Instance with Duo Pro license on a group

0. Run GITLAB_SIMULATE_SAAS=1 gdk restart in your GDK.
1. Go to Groups > Settings > GitLab Duo in the left nav.
2. Under Seat utilization, select the Assign seats button.
3. Under the Subscription box on the right hand side, validate the button now says Purchase seats with an external link icon.

Related to #507331 (closed)