feat(catalog-react): migrate UnregisterEntityDialog to Backstage UI

Migrated UnregisterEntityDialog from Material UI to Backstage UI components.

- Replaced MUI Dialog, Button, Alert, Box with BUI equivalents
- Refactored to hook-based architecture with separate body components
- Added proper button loading states during async actions
- Extracted shared AdvancedDeleteAccordion component
- No breaking changes to public API

Signed-off-by: Johan Persson <johanopersson@gmail.com>

yarn.lock

@@ -5400,6 +5400,7 @@ __metadata:
     "@backstage/plugin-scaffolder-common": "workspace:^"
     "@backstage/test-utils": "workspace:^"
     "@backstage/types": "workspace:^"
+    "@backstage/ui": "workspace:^"
     "@backstage/version-bridge": "workspace:^"
     "@material-ui/core": "npm:^4.12.2"
     "@material-ui/icons": "npm:^4.9.1"