Remove manage subscription button for Customers Portal button

What does this MR do and why?

Resolves part of #374938 (closed)

Removes the Manage subscription button and adds a customers portal button in the top right corner.

Screenshots or screen recordings

Before	After

CustomerPortal link (for invalid license)	CustomerPortal link (for valid license)

How to set up and validate locally

• Have CustomersDot set up and running with an active license
• Navigate to /admin/subscription on GitLab
• Verify that the Manage button no longer exists in the Subscription Details table
• Verify that the Customers Portal button exists and matches the Figma link
• Verify that the Customers Portal button opens CustomersDot

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.

Related to #374938 (closed)