fix(dashboard): Replace react-nestable with new SortableTree component (#8599)

**What**
- Removes `react-nestable` dependency in favour of our own solution based on `@dnd-kit/core`

Resolves CC-217

packages/admin-next/dashboard/package.json

@@ -58,7 +58,6 @@
     "react-hook-form": "7.49.1",
     "react-i18next": "13.5.0",
     "react-jwt": "^1.2.0",
-    "react-nestable": "^3.0.2",
     "react-router-dom": "6.20.1",
     "zod": "3.22.4"
   },