Pin tailwind 3.x for renovate

Cf. !13
Cf. https://github.com/rails/tailwindcss-rails?tab=readme-ov-file#upgrading-your-application-from-tailwind-v3-to-v4

renovate.json

@@ -21,6 +21,7 @@
         "ghcr.io/rails/devcontainer/images/ruby"
       ],
       "groupName": "ruby"
-    }
+    },
+    {"matchPackageNames": ["tailwindcss-rails"], "allowedVersions": "<4.0"}
   ]
 }