[12.x] Add Model::withoutRelation() for selective relation unloading

[SanderMuller]

Adds withoutRelation(string|array $relations) which clones the model and unsets only the specified relations, leaving the rest intact.

Motivation

Laravel provides withoutRelations() to clone a model without any loaded relations, and unsetRelation() to
mutate a model in place. But there's no non-mutating way to selectively remove specific relations:

// Remove all relations (clone) — exists
$clean = $model->withoutRelations();

// Remove one relation (mutates) — exists
$model->unsetRelation('comments');

// Remove specific relations without mutating — missing
$lightweight = $model->withoutRelation('comments');

This comes up when passing models to contexts where certain relations cause issues: circular references during serialization, unnecessary memory overhead in queued jobs, or Livewire/Inertia payloads where you want to strip heavy relations before sending to the frontend:

$block->setRelation('overview', $this->withoutRelation('blocks'));

Test plan

• Single relation: clones model, removes only the specified relation, original unchanged
• Array of relations: removes multiple relations selectively, keeps the rest
• Existing withoutRelations() behavior unchanged

[shaedrich]

While we use withers all across the framework, I wonder if it could be confused with with() and without() 🤔