[8.x] Modify mergeCasts to return $this

[finagin]

RP #35677 for 8.x branch

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()]);
    }

P.S.

@GrahamCampbell this isn't a breaking change
#35676 (comment)

[GrahamCampbell]

Changing a void method to a non-void method in a non-final class is a breaking change. Anyone extending this class now fails to return the correct thing.

[driesvints]

@GrahamCampbell there's no type annotation on the method. This breaks nothing in real code.

[dinhquochan]

Yes, i agree @driesvints

[afraca]

If you have CI which does static analysis and fails hard (cannot deploy) when your model differs from base model I would call it breakage. I know this is far fetched, just saying things could break from some people.
https://xkcd.com/1172/

[driesvints]

@afraca no actual code is broken. We don't optimize for static analysis tools.