packaging: add Bullseye, remove Jessie

Jessie is oldoldstable now, and Bullseye is going to be the next stable (now
testing). We're continuing to support the current oldstable, stable and testing
releases.

Differential Revision: https://phab.mercurial-scm.org/D6762

contrib/packaging/Makefile

@@ -1,9 +1,9 b''
 $(eval HGROOT := $(shell cd ../..; pwd))
 
 DEBIAN_CODENAMES := \
-  jessie \
   stretch \
-  buster
+  buster \
+  bullseye
 
 UBUNTU_CODENAMES := \
   xenial \