Clarify conformance language re search via POST and GET:

 SHALL also support a POST based search:

Is interpreted to mean POST and GET SHALL be supported by the server.

This discussion clarifies that it is not the case:  https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Search.20via.20POST

The intent is that servers SHALL support POST and SHOULD support GET

Suggested  text to clarify:

The interaction can be performed by several different HTTP commands.  Servers that support search _SHOULD support search via GET method._

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 Servers t_hat support search_ SHALL support search via POST method.

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