Create unstable-proposed-updates suite for use during freeze

Problem

Currently uploading new upstream versions to unstable during freeze is discouraged. It means users using unstable don't get new updates and developers are forced to upload to experimental. Using experimental directly is risky as it can have changes not ready for unstable also.

Proposed solution

Open unstable-proposed-updates with freeze and close it when freeze is lifted. The packages in this suite can migrate to testing, this avoids manual reuploads to unstable after freeze is lifted.

Optional: create a companion testing suite say rolling which may be used during this time and a second Britney instance can manage migration to this suite.

This was discussed in https://lists.debian.org/debian-devel/2021/08/msg00173.html