d/gbp.conf: Adjust branch name to 'pureos/latest'

Using 'pureos/latest' is the current recommendation due
https://tracker.pureos.net/w/development/packaging_overview

debian/gbp.conf

 [DEFAULT]
-debian-branch = pureos/master
+debian-branch = pureos/latest
 debian-tag = pureos/%(version)s
 upstream-branch =
 upstream-tag =