Fix image tags that failed to pull on test-cloud

- discourse: 2026.10.0 -> 2026.10.0-latest (correct tag format)
- formbricks: Docker Hub 6.1.0 -> ghcr.io 4.6.0 (moved to GHCR)
- zulip: Docker Hub 12.3-0 -> ghcr.io 12.3-0 (12.x on GHCR)
- open-webui: v0.12.3 -> v0.12.4 (newer release)

zulip/versions/12/deployment.yaml

@@ -22,7 +22,7 @@ spec:
           type: RuntimeDefault
       containers:
         - name: zulip
-          image: zulip/docker-zulip:12.3-0
+          image: ghcr.io/zulip/zulip-server:12.3-0
           ports:
             - name: http
               containerPort: 80