Add delete upstream action to upstream list view

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Context

Add an ellipsis icon button to the Upstream List view, 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

• Left over from Maven virtual registry Upstreams list view (#556837)
• Supports discoverability of the delete action directly from the list view
• Allows consistent access to deletion from multiple entry points