Use international extension for timezone

[lmckenzi]

The au base guide defines http://hl7.org.au/fhir/StructureDefinition/au-timezone|4.1.1
However, there's already a standard extension http://hl7.org/fhir/StructureDefinition/timezone|1.0.0 which seems to serve the same purpose and should be used instead

[brettesler-ext]

FHIR-46319