Show associated registries before Maven upstream deletion

Context

The ability of adding an upstream to a maven virtual registry by referencing an existing upstream, basically shareable upstreams was implemented in Maven virtual registry: shareable upstreams (#541117 - closed)

Originally a Select upstreams dropdown was designed in #493842[New-registry.png] (comment 2631348123)

We want to revise the experience based on #545992 (comment 2634872489)

Solution

The revised experience would look like this:

1. The Create Registry form includes only fields related to the registry itself (name, optional description).
2. After saving, users are taken to the Registry Details page, where they can:
   • Link existing upstreams
   • Or create new ones (If no upstreams exist yet, only the "Create upstream" action is shown.)

Deletion vs. Unlinking upstreams

The core concern is that users may not realise deleting a shared upstream affects multiple registries. When reusing an upstream, users are likely thinking in terms of "detach", not "destroy".

Proposal is:

• Add a badge (e.g., Shared or Used by 3 registries) with a tooltip to signal that the upstream is shared.
• Show associated registries in the Upstream Details page and also in the Upstream Listing page.
• Allowing deletion, but gating it with a confirmation modal that clearly shows:
  • The number of registries using the upstream
  • Their names
  • A description of the impact before proceeding

Updated implementation plan

In this issue we will only deal with adding extra warning for the user before deleting the upstream.

In Edit upstream page, update the confirmation modal which is shown upon clicking the Delete upstream button to show:

• The number of registries using the upstream
• Their names
• A description of the impact before proceeding

For this we need the list of associated registries for the upstream from the backend