GroupRelationship::postDelete can try and save a deleted entity

Problem/Motivation

When deleting a group relationship the postDelete method retrieves the entity referenced from the relationship entity https://git.drupalcode.org/project/group/-/blob/1d7b4ee7d69b3bfcfde8cc79...
However if prior to this call the reference has been used to get the entity for whatever reason (just one example see steps to reproduce) it will be on the relationship entity even if in the database the referenced entity has since been deleted.

Steps to reproduce

Code I was using that stumbled on this was finding the entities to delete from their references. So:

$relationship = GroupRelationship::load($id);
$entity = $relationship->getEntity();
$entity->delete();

Run with $id being a gnode will cause errors:

[error]  Drupal\Core\Entity\EntityStorageException: Update existing 'node' entity while changing the ID is not supported. in Drupal\Core\Entity\ContentEntityStorageBase->doPreSave() (line 762 of /app/web/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php). 
 [error]  Drupal\Core\Entity\EntityStorageException: Update existing 'node' entity while changing the ID is not supported. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (line 815 of /app/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php). 
 [error]  Drupal\Core\Entity\EntityStorageException: Update existing 'node' entity while changing the ID is not supported. in Drupal\Core\Entity\Sql\SqlContentEntityStorage->delete() (line 761 of /app/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php). 

As it tries to resave the already deleted entity.

Proposed resolution

Reloading the entity to check that it still exists works.

Remaining tasks

Patch/MR to follow.