Add return-void annotations

[rrehbein]

Add return type hints for void-return methods.

Some of the tooling I'm using was emitting:

Method "League\OAuth2\Client\Provider\AbstractProvider::checkResponse()" might add "void" as a native return type declaration in the future. Do the same in child class "CakeDC\OAuth2\Client\Provider\Cognito" now to avoid errors or add an explicit @return annotation to suppress this message.

[rrehbein]

It appears the project has a minimum PHP 7.3, which supports adding void as a return type, however the current code style appears to not use this feature. If interested, I could add the return types to this PR as well, or would that be against the project's code style?