Debian: we want Openvswitch and ovn from backports

Those packages are in exactly same versions in bullseye-xena-backports
and in bullseye-backports. So we have to choose one source to keep
builds running otherwise they report downgrading of packages.

Change-Id: I0954bcc8ee42b15420fe1cd54aac96cb0c0c7e0d
(cherry picked from commit a26f4e3)

Author: hrw

docker/base/apt_preferences.debian

@@ -17,3 +17,13 @@ Pin-Priority: 1000
 Package: logstash-oss
 Pin: version 7.9.*
 Pin-Priority: 1000
+
+# NOTE(hrw): we do not want backports unless requested
+Package: *
+Pin: release n=bullseye-backports
+Pin-Priority: -1000
+
+# NOTE(hrw): let us list some backports
+Package: openvswitch* python3-openvswitch ovn-*
+Pin: release n=bullseye-backports
+Pin-Priority: 1024