Need to lock down code system versions in value sets that are schema bound

[lmckenzi]

The value sets that we generate and publish that are bound to 'code' elements in the specification AND where we enumerate the allowed set of codes in the schema (i.e. not bound to value sets like "all mime types") need to have the following changes:

• the reference to the code system needs to be version-specific. E.g. <system value="http://hl7.org/fhir/administrative-gender|4.0.1"/>, not <system value="http://hl7.org/fhir/administrative-gender"/>
• those valuesets must NOT be flagged as "immutable" because the version will need to change each time a new release is published

Reviewed and approved by FHIR-I on 2022-01-20