Warnings with the new SUSHI version

[qligier]

As per FHIR/sushi#1620, there will be new warnings from the SUSHI generation:

warn  Found old-style cross-version extensions package in dependencies: hl7.fhir.extensions.r5.
  SUSHI will use the official xver package instead: hl7.fhir.uv.xver-r5.r4.
  Please update your sushi-config.yaml file to reference the new xver package.
  See: https://confluence.hl7.org/spaces/FHIRI/pages/413256623/FAQs
warn  Cross-version extensions for choice elements should omit the [x] suffix.
  Found URL:     http://hl7.org/fhir/5.0/StructureDefinition/extension-Observation.value[x]
  Corrected URL: http://hl7.org/fhir/5.0/StructureDefinition/extension-Observation.value
  SUSHI will use the corrected URL, but authors should fix the URL in their FSH source.
warn  Cross-version extensions for choice elements should omit the [x] suffix.
  Found URL:     http://hl7.org/fhir/5.0/StructureDefinition/extension-Observation.value[x]
  Corrected URL: http://hl7.org/fhir/5.0/StructureDefinition/extension-Observation.value
  SUSHI will use the corrected URL, but authors should fix the URL in their FSH source.