Switch from json to json_pure

[fulldecent]

What was previously known as json, a gem used here, is now called json_pure.

And the json gem was changed be a high-performance binary module.

Don't we want high performance? Well we don't use a ton of JSON processing here, so switching to the json_pure package will be easier for people to use. For example, as current w3c_validators cannot be used on Apple+DevContainers platforms, discussed at ruby/rubygems#6212