As mentioned here:

Instead of using those shortcuts, inject the related services in the constructor or the controller methods.

You need to use dependency injection.

For a given controller, simply inject ManagerRegistry on the controller's constructor.

use Doctrine\Persistence\ManagerRegistry;

class SomeController {

    public function __construct(private ManagerRegistry $doctrine) {}

    public function someAction(Request $request) {
        // access Doctrine
        $this->doctrine;
    }
} 

You can use EntityManagerInterface $entityManager:

public function delete(Request $request, Test $test, EntityManagerInterface $entityManager): Response
{
    if ($this->isCsrfTokenValid('delete'.$test->getId(), $request->request->get('_token'))) {
        $entityManager->remove($test);
        $entityManager->flush();
    }

    return $this->redirectToRoute('test_index', [], Response::HTTP_SEE_OTHER);
}

As per the answer of @yivi and as mentionned in the documentation, you can also follow the example below by injecting Doctrine\Persistence\ManagerRegistry directly in the method you want:

// src/Controller/ProductController.php
namespace App\Controller;

// ...
use App\Entity\Product;
use Doctrine\Persistence\ManagerRegistry;
use Symfony\Component\HttpFoundation\Response;

class ProductController extends AbstractController
{
    /**
     * @Route("/product", name="create_product")
     */
    public function createProduct(ManagerRegistry $doctrine): Response
    {
        $entityManager = $doctrine->getManager();

        $product = new Product();
        $product->setName('Keyboard');
        $product->setPrice(1999);
        $product->setDescription('Ergonomic and stylish!');

        // tell Doctrine you want to (eventually) save the Product (no queries yet)
        $entityManager->persist($product);

        // actually executes the queries (i.e. the INSERT query)
        $entityManager->flush();

        return new Response('Saved new product with id '.$product->getId());
    }
}