Fatal error due to using obsolete interface in the actions

Thank you for your contribution

Commit 6a90ca0b broke my install. When I click the "Approve membership" button, it displays the following fatal error:

The website encountered an unexpected error. Please try again later.

RuntimeException: Controller "Drupal\grequest\Controller\GroupMembershipRequestController::approveMembership()" requires that you provide a value for the "$group_relationship" argument. Either the argument is nullable and no null value has been provided, no default value has been provided or because there is a non optional argument after this one. in Symfony\Component\HttpKernel\Controller\ArgumentResolver->getArguments() (line 80 of /app/vendor/symfony/http-kernel/Controller/ArgumentResolver.php).
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->onController(Object, 'kernel.controller', Object)
call_user_func(Array, Object, 'kernel.controller', Object) (Line: 142)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch(Object, 'kernel.controller') (Line: 157)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

```

Reverting this commit fixes the problem. I think there may be some other places where the group_content needs to be renamed to group_relationship, such as in the routing parameters for example:
docroot/modules/contrib/grequest/grequest.routing.yml

My fault, I will tag a new version

Automatically closed - issue fixed for 2 weeks with no activity.