Stabilize translation API in the new frontend system

Remove @alpha tags from translation-related types and switch all
translation API imports in new frontend system packages to use stable
paths from @backstage/frontend-plugin-api instead of
@backstage/core-plugin-api/alpha.

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

packages/frontend-test-utils/src/apis/TranslationApi/MockTranslationApi.test.tsx

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { createTranslationRef } from '@backstage/core-plugin-api/alpha';
+import { createTranslationRef } from '@backstage/frontend-plugin-api';
 import { MockTranslationApi } from './MockTranslationApi';
 
 describe('MockTranslationApi', () => {

packages/frontend-test-utils/src/apis/TranslationApi/MockTranslationApi.ts

@@ -18,7 +18,7 @@ import {
   TranslationApi,
   TranslationRef,
   TranslationSnapshot,
-} from '@backstage/core-plugin-api/alpha';
+} from '@backstage/frontend-plugin-api';
 import { createInstance as createI18n, type i18n as I18n } from 'i18next';
 import ObservableImpl from 'zen-observable';
 
@@ -32,7 +32,7 @@ import { JsxInterpolator } from '../../../../core-app-api/src/apis/implementatio
 const DEFAULT_LANGUAGE = 'en';
 
 /**
- * Mock implementation of {@link @backstage/core-plugin-api/alpha#TranslationApi}.
+ * Mock implementation of {@link @backstage/frontend-plugin-api#TranslationApi}.
  *
  * @public
  */

packages/frontend-test-utils/src/apis/mockApis.ts

@@ -201,7 +201,7 @@ export namespace mockApis {
   }
 
   /**
-   * Fake implementation of {@link @backstage/core-plugin-api/alpha#TranslationApi}.
+   * Fake implementation of {@link @backstage/frontend-plugin-api#TranslationApi}.
    * By default returns the default translation.
    *
    * @public
@@ -216,14 +216,14 @@ export namespace mockApis {
   }
 
   /**
-   * Mock helpers for {@link @backstage/core-plugin-api/alpha#TranslationApi}.
+   * Mock helpers for {@link @backstage/frontend-plugin-api#TranslationApi}.
    *
    * @see {@link @backstage/frontend-plugin-api#mockApis.translation}
    * @public
    */
   export namespace translation {
     /**
-     * Creates a mock of {@link @backstage/core-plugin-api/alpha#TranslationApi}.
+     * Creates a mock of {@link @backstage/frontend-plugin-api#TranslationApi}.
      *
      * @public
      */