Move ExtensionFactoryMiddleware from frontend-plugin-api to frontend-app-api

The `ExtensionFactoryMiddleware` type is only used by the app-level wiring
and has been moved to `@backstage/frontend-app-api` as its canonical location.

The type is preserved in `@backstage/frontend-plugin-api` with a `@deprecated`
tag pointing to the new location. All internal usages have been updated to
import from the new location.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

packages/frontend-defaults/report.api.md

@@ -8,7 +8,7 @@ import { AppErrorTypes } from '@backstage/frontend-app-api';
 import { Config } from '@backstage/config';
 import { ConfigApi } from '@backstage/frontend-plugin-api';
 import { CreateAppRouteBinder } from '@backstage/frontend-app-api';
-import { ExtensionFactoryMiddleware } from '@backstage/frontend-plugin-api';
+import { ExtensionFactoryMiddleware } from '@backstage/frontend-app-api';
 import { FrontendFeature } from '@backstage/frontend-plugin-api';
 import { FrontendFeatureLoader } from '@backstage/frontend-plugin-api';
 import { FrontendPluginInfoResolver } from '@backstage/frontend-app-api';

packages/frontend-defaults/src/createApp.tsx

@@ -18,7 +18,6 @@ import { JSX, lazy, ReactNode, Suspense } from 'react';
 import {
   ConfigApi,
   coreExtensionData,
-  ExtensionFactoryMiddleware,
   FrontendFeature,
   FrontendFeatureLoader,
 } from '@backstage/frontend-plugin-api';
@@ -31,6 +30,7 @@ import { ConfigReader } from '@backstage/config';
 import {
   CreateAppRouteBinder,
   createSpecializedApp,
+  ExtensionFactoryMiddleware,
   FrontendPluginInfoResolver,
 } from '@backstage/frontend-app-api';
 import appPlugin from '@backstage/plugin-app';