ci: Fix failing Ubuntu 22.04 job.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

Author: andersk

.github/workflows/zulip-ci.yml

@@ -28,7 +28,7 @@ jobs:
             name: Ubuntu 22.04 (Python 3.10, backend)
             os: jammy
             legacy_client_interface: "6"
-            server_version: refs/tags/6.0
+            server_version: refs/pull/36159/head
           # Debian 12 ships with Python 3.11.2.
           - docker_image: zulip/ci:bookworm
             name: Debian 12 (Python 3.11, backend + documentation)