Remove deprecated json/add/* types

[byroot]

I honestly don't know much about RBS, so unsure what exactly should be done.

But all these json/add/*.rb methods are gone in JSON 3.x: https://github.com/ruby/json/releases/tag/v3.0.0.rc1

So it's causing issues on ruby's master branch: ruby/ruby#18295

[soutaro]

Thanks @byroot!
I think skipping tests with JSON 3.x would be better because the RBS tests should run with released versions of Ruby with JSON 2.x. #3084

[byroot]

Up to you, I don't have much context on RBS.