[9.x] Modify mergeCasts to return $this

[finagin]

This change helpful to use the method in pipeline like this:

    public function initializeUsesUuid()
    {
        $this
            ->setKeyName('uuid')
            ->setKeyType('string')
            ->setIncrementing(false)
            ->mergeCasts([$this->getKeyName() => $this->getKeyType()])
            ->mergeGuarded([$this->getKeyName()]);
    }

[dinhquochan]

@taylorotwell Can merge this into 8.x? Because It's not breaking changes (not much).

// before
public function initializeUsesUuid()
{
        $this->setKeyType('string')
            ->setIncrementing(false);
        $this->mergeCasts([$this->getKeyName() => $this->getKeyType()]);
}

// after
public function initializeUsesUuid()
{
        $this->setKeyType('string')
            ->setIncrementing(false)
            ->mergeCasts([$this->getKeyName() => $this->getKeyType()]);
}

[driesvints]

@dinhquochan feel free to pr

[finagin]

@driesvints, hey, I create this PR to 8.x (#35676), but it's rejected

[driesvints]

@finagin that PR shouldn't have been closed. Feel free to resubmit it.

[finagin]

@driesvints I can't reopen #35676 but open new PR #35683