use 5.0-rc1-0 tag from official zulip docker repo

flux2/apps/zulip/zulip-values-configmap.yaml

@@ -7,10 +7,7 @@ metadata:
 data:
   values.yaml: |
     image:
-      # Overrides the image to a version on Zulip's `main` branch because the
-      # most recent stable version (4.9) does not support OIDC (we need 5)
-      repository: open.greenhost.net:4567/stackspin/stackspin/zulip
-      tag: "5e90b32f40"
+      tag: "5.0-rc1-0"
 
     ingress:
       enabled: true