Use DEQM populationDescription extension

[brynrhodes]

The evaluate-measure operation currently uses a pre-adoption extension of the R6 population description element. However, since R6 isn't published yet, that pre-adoption extension doesn't actually work. There is a tracker against DEQM to add an R4 extension to support population description (https://jira.hl7.org/browse/FHIR-43324), we should use that extension, rather than the pre-adoption extension:

Note that this is effectively blocked until at least a disposition is reached on 43324.