sysadm: Drop stretch references from documentation

It's no longer built for it.

sysadm/deployment/howto-debian-packaging.rst

@@ -52,8 +52,7 @@ For packages to be accepted, they need to be :
 #. A changes file uploaded to ``/srv/softwareheritage/repository/incoming``:
 
 #. Targeted at one of the supported distributions (unstable, unstable-swh, buster,
-   buster-backports, buster-backports-swh), stretch, stretch-backports,
-   stretch-backports-swh)
+   buster-backports, buster-backports-swh)
 
 #. Signed by one of the keys listed in
    ``/srv/softwareheritage/repository/conf/uploaders``