Require all implementers to support at least one common JSON Schema version

[decentralgabe]

@msporny has suggested requiring implementers to support at least one common JSON Schema version. I like the idea as it makes testability easier, and ensures broader success in adoption.

To achieve this text in the processing section should be updated from...

It is RECOMMENDED that implementers choose validators which possess this capability and are able to limit validation to the JSON schema specifications supported by this document.

to something like

It is REQUIRED that implementers choose validators which possess this capability and are able to limit validation to the JSON schema specifications supported by this document.

with the addition of language specifying which version all implementers must support.

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As a separate, but related concern when the JSON Schema organization updates to a stable version identifier for long term support, we should require that version.