Make createBackendPlugin and createBackendModule return BackendFeature instead of () => BackendFeature

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

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

@@ -3,10 +3,10 @@
 > Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
 
 ```ts
-import { BackendFeature } from '@backstage/backend-plugin-api';
+import { BackendFeatureCompat } from '@backstage/backend-plugin-api';
 
 // @alpha
-const _default: () => BackendFeature;
+const _default: BackendFeatureCompat;
 export default _default;
 
 // (No @packageDocumentation comment for this package)