apt is not going to automatically follow the recommended upgrade path as documented, you will need to manually specify which package version to upgrade to.
I was unable to find a way to upgrade via interim versions. The only packages available via apt where 5,1 and 5.2alpha. However the the 3.2 → 5.1 worked no problem.
You need to first upgrade elasticsearch also. Upgrading elastic search works no problem, but you must manually delete the Elasticsearch indexes for Zammad and then reindex otherwise Zammad will not start because the new Elasticsearch 7.1 will not read the earlier index unless you are already at version 6+
Once I worked out where the indexes where it was no problem to delete and reindex.