Unpin publisher back to `latest`

[iyt-trifork]

In the IG publisher 2.0.16, a bug was introduced: https://chat.fhir.org/#narrow/channel/215610-shorthand/topic/The.20type.20Reference.20does.20not.20match.20any.20of.20the.20allowed.20types/with/541219374

A fix is expected in HAPI 8.6.0: hapifhir/hapi-fhir#7235

For the time being we have downgraded the publisher version.