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

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

plugins/app-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 appPlugin: () => BackendFeature;
+const appPlugin: BackendFeatureCompat;
 export default appPlugin;
 
 // (No @packageDocumentation comment for this package)

plugins/app-backend/src/service/appPlugin.test.ts

@@ -47,7 +47,7 @@ describe('appPlugin', () => {
   it('boots', async () => {
     const { server } = await startTestBackend({
       features: [
-        appPlugin(),
+        appPlugin,
         mockServices.rootConfig.factory({
           data: {
             app: {