Fix CI: downgrade public_suffix to 5.1.1 compatible with Ruby 2.7.3

Agent-Logs-Url: https://github.com/github/catalyst/sessions/4c09a1ac-5395-4e68-bd72-fbfc3ec24679

Co-authored-by: liuliu-dev <6688812+liuliu-dev@users.noreply.github.com>

Author: Copilot

docs/Gemfile.lock

@@ -80,7 +80,7 @@ GEM
       sawyer (~> 0.8.0, >= 0.5.3)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (7.0.5)
+    public_suffix (5.1.1)
     rb-fsevent (0.11.1)
     rb-inotify (0.10.1)
       ffi (~> 1.0)