fix: update message for missing Electron main entries (#13242)

The warning message for missing Electron main entries still refered to
'src-gen/frontend'. However since Theia 1.39.0 it should be either
'src-gen/backend' or as now indicated 'lib/backend'.

dev-packages/application-manager/src/application-package-manager.ts

@@ -139,7 +139,7 @@ export class ApplicationPackageManager {
             console.warn(
                 `WARNING: ${this.pck.packagePath} does not have a "main" entry.\n` +
                 'Please add the following line:\n' +
-                '    "main": "src-gen/frontend/electron-main.js"'
+                '    "main": "lib/backend/electron-main.js"'
             );
         }