D11.3 Deprecation warning re getUserEntityByIdentifier() missing @return

Problem/Motivation

Running tests on an unrelated custom module I get the following warning: vendor/symfony/error-handler/DebugClassLoader.php:363
Method "OpenIDConnectServer\Repositories\IdentityProviderInterface::getUserEntityByIdentifier()" might add "UserEntityInterface&ClaimSetInterface" as a native return type declaration in the future. Do the same in implementation "Drupal\simple_oauth\OpenIdConnect\UserIdentityProvider" now to avoid errors or add an explicit @return annotation to suppress this message.

Steps to reproduce

Run a test on a contrib or custom module.

Proposed resolution

I'll post an untested MR in a bit.

Remaining tasks

Test the MR, or decide if you want to do this another way.