Set created Merge Requests to unassigned

Gitlab now support merge_request.unassign=
This fits in well with our workflow to have all MRs unassigned by
default.

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -106,6 +106,7 @@ def push_merge_request(project_url, proposed_branch, upstream, downstream, auto_
   git_push_custom(
     "-o", "merge_request.create",
     "-o", "merge_request.remove_source_branch",
+    "-o", "merge_request.unassign=",
     "-o", f"merge_request.target={downstream}",
     "-o", f"merge_request.title={title}",
     project_url,