Support using FHIR R6 in SUSHI

Transferred from internal JIRA issue originally created 2024-01-18.

SUSHI should be able to build an IG that uses R6 (currently `6.0.0-ballot1`). We’ll need to first be able to load R6 as a FHIR version. Once we can load R6, this may also require looking at any changes to StructureDefinition, ElementDefinition, ValueSet, CodeSystem, etc. and making sure there aren’t changes to things that SUSHI relies on.

SUSHI seems to be the one limiting the FHIR versions that can be used (here) and builds the package name and version to pass to FPL to load. However, there may still be changes in FPL that are required to properly load the R6 package.

Start by locating the SUSHI code that limits supported FHIR versions and constructs the package name and version passed to FPL. Then investigate loading the FHIR R6 6.0.0-ballot1 package and check whether changes to StructureDefinition, ElementDefinition, ValueSet, or CodeSystem require SUSHI or FPL updates; done means an R6-based IG builds successfully.

Written by the indexing model from the issue text.