ci: fix Chromium sandbox issue of manager web test (#2317)

This PR resolves the [sandbox error encountered during server manager web tests](https://github.com/Jigsaw-Code/outline-apps/actions/runs/12438976622/job/34995431528?pr=2291):

> `Cannot start ChromiumHeadless: No usable sandbox!`

This error is caused by [the more restricted unprivileged user namespaces feature](https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces) introduced in Ubuntu 23. It's a known limitation and many chromium based products (including electron) are affected by this change.

.github/workflows/build_and_test_debug_manager.yml

@@ -32,7 +32,7 @@ on:
 jobs:
   web_test:
     name: Web Test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout
         uses: actions/[email protected]