Add delete upstream action to upstream detail page

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Context

Add an ellipsis icon button to the Upstream Detail page, allowing users to access the Delete upstream action.

Proposal

• Add an ellipsis icon button to the page
• Include a menu item Delete upstream
• Trigger the same confirmation modal in #556828 (closed)

Why

• Supports discoverability of the delete action directly from the detail view
• Aligns with delete action patterns used across GitLab (e.g., Package Registry Detail pages, User Detail, etc)
• Allows consistent access to deletion from multiple entry points

Considerations

1. When an upstream is deleted, we need to redirect users back to the list view. !191279 (comment 2509975444)