UM only: change zed to use unmaintained/

As [1] merged stable/zed transformed to unmaintained/zed,
so the related jobs must be adopted.

[1]: https://review.opendev.org/c/openstack/releases/+/916492

Change-Id: I1df544fc9c37d2fea070775855e0e8d7c00801a9
1 file changed