The injector getter is annotated with @internal and is removed from declarations.
This commit removes the annotation thus it's possible to access the injector on the
ApplicationRef, e.g., after calling bootstrapApplication.

Currently, it requires casting ApplicationRef to { injector: Injector } after calling the bootstrapApplication, e.g.:

bootstrapApplication(...).then((appRef) => {
  const injector = (appRef as unknown as { injector: Injector }).injector;
  const router = injector.get(Router);
  ...
});

NgModuleRef prior to ApplicationRef has a public injector property which allows the above behavior:

platformBrowserDynamic()
  .bootstrapModule(...)
  .then((ngModuleRef) => {
    const router = ngModuleRef.injector.get(Router);
    ...
  });

This change also allows a more straightforward setup for micro-frontend adapters (e.g., single-spa-angular). The micro-frontend bootstrap function can resolve to ApplicationRef | NgModuleRef<T>; they both have the destroy method now, and having injector on both types doesn't require type cast for ApplicationRef.