[6.x] Implement withoutRelations method

[driesvints]

This will allow to unload all of the model's relations so it can be used, for example, to not overload queued jobs. This allows for example to do this in a queued job:

protected $podcast;

public function __construct(Podcast $podcast)
{
    $this->podcast = $podcast->withoutRelations();
}

This way those queued jobs won't be overloaded. I'll document this once it's merged.

Fixes #27148