Move towards SHALL support JSON, SHOULD support XML

[james-answer]

Change wording in spec to move away from dual support and towards only JSON support as per HDL ticket:

HDL-181

Affects:

https://nhsconnect.github.io/gpconnect/development_fhir_api_guidance.html#wire-format-representations

and possibly various other points through the spec (need to do a grep search through content)

We will still test and validate both, but will not require suppliers to support both.

[james-answer]

I have updated the wording in all the places that refer to JSON and XML types to clearly state that JSON is SHALL and XML is SHOULD.