fix(admin): Use Customer details endpoint rather than deprecated subscription details

[kcons]

The subscription-based path has been considered deprecated for 9 years, since 7c83890.

[sentry]

Bug: The migration from the deprecated /subscriptions/ endpoint to /customers/ is incomplete. usePerformanceSubscriptionDetails.tsx still uses the old endpoint, which will cause API failures on performance and explore pages.
_{Severity: HIGH}

Suggested Fix

Update the API endpoint in static/app/views/performance/newTraceDetails/traceTypeWarnings/usePerformanceSubscriptionDetails.tsx to use /customers/${organization.slug}/ instead of /subscriptions/${organization.slug}/. Also, search for and update any remaining test mocks that still use the deprecated endpoint.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: static/gsApp/stores/subscriptionStore.tsx#L110

Potential issue: The pull request updates an API endpoint from the deprecated
`/subscriptions/${orgSlug}/` to `/customers/${orgSlug}/`. However, this migration is
incomplete. The file
`static/app/views/performance/newTraceDetails/traceTypeWarnings/usePerformanceSubscriptionDetails.tsx`
was not updated and still references the old endpoint. This hook is actively used in
performance trace details and explore pages. As a result, users accessing these features
will encounter API failures, as the frontend will attempt to fetch data from a
deprecated or non-existent endpoint, leading to broken functionality.

_{Did we get this right? 👍 / 👎 to inform future reviews.}