Update dependencies

[laurynasr]

Update dependencies to use newest JsonSchema.Net

I tried following the pattern in JsonSchema repository for keyword changes, and also fixed anything that was causing build breaks:

• Json.More does not provide JsonNode.Copy any more -- replaced it with native JsonNode.DeepClone
• JsonNull was replaced by .net's null
• JsonPointer does not have .Segments property any more and itself is an enumerable of segments
• Use WeaklyTypedJsonConverter for all keywords, following the pattern in JsonSchema.Net
• Lazily initialize Draft4MetaSchema as keywords need to be registered before its initialization
• I am not entirely sure why NullableKeywordJsonConverter change was required, but without that it was serializing nullable as "nullable": "nullable": true

[laurynasr]

@gregsdennis sorry I did not notice your PR, I'll close this one