Update release distributions. (#236)

* Drop Ubuntu Xenial which ended public support in April 2021
* Drop Debian Stretch which stopped receiving security updates in July
  2021
* Add Ubuntu Jammy which is the forthcoming LTS in April 2022
* Add Debian Bullseye which is the current stable release of Debian

Author: nuclearsandwich

publish-python.yaml

@@ -8,8 +8,8 @@ artifacts:
         config:
           repository: dirk-thomas/vcstool
           distributions:
-            - ubuntu:xenial
             - ubuntu:bionic
             - ubuntu:focal
-            - debian:stretch
+            - ubuntu:jammy
             - debian:buster
+            - debian:bullseye