Passing a boolean to the constructor of “Symfony\Component\Dotenv\Dotenv” is deprecated

You might encounter this deprecation message after your upgrade to symfony 5.1 in your symfony project:

Since symfony/dotenv 5.1: Passing a boolean to the constructor of “Symfony\Component\Dotenv\Dotenv” is deprecated, use “Dotenv::usePutenv()

To fix this, go to config/bootstrap.php and remove the false from the Dotenv constructor:
(new Dotenv(false))->loadEnv(dirname(DIR).'/.env');
(new Dotenv())->loadEnv(dirname(DIR).'/.env');

Actually the file config/bootstrap.php has been removed from symfony 5.1 and was replaced in public/index.php.

But since the project was updated from an symfony 4 version the bootrap file is still present.

So another option would be to remove the boostrap file and update the project entirely to symfony 5.1 recipe.
This is not trivial though as you can see in this corresponfing PullRequest.