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)

formbricks/versions/6/manifest.yaml

@@ -1,4 +1,4 @@
-version: 6.1.0
+version: 4.6.0
 requires:
 - name: postgres
 - name: redis