plugins: refactory to use default export for new backend system

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Camila Belo <camilaibs@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Co-authored-by: Philipp Hugenroth <philipph@spotify.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/app-backend/alpha-api-report.md

@@ -6,7 +6,8 @@
 import { BackendFeature } from '@backstage/backend-plugin-api';
 
 // @alpha
-export const appPlugin: () => BackendFeature;
+const appPlugin: () => BackendFeature;
+export default appPlugin;
 
 // (No @packageDocumentation comment for this package)
 ```

plugins/app-backend/src/alpha.ts

@@ -14,4 +14,4 @@
  * limitations under the License.
  */
 
-export { appPlugin } from './service/appPlugin';
+export { appPlugin as default } from './service/appPlugin';