Follow nginx mainline again

blog and czsk now run nginx mainline (1.31), which upstream recommends;
the stable-only rule would hold their updates back. Drop it.

Assisted-by: Claude:opus-5-5

default.json

@@ -43,14 +43,6 @@
         "A Postgres major upgrade needs pg_dumpall + restore into a fresh volume (see storage/README.md). Do not merge as a routine bump."
       ]
     },
-    {
-      "description": "nginx odd minors (1.29, 1.31) are mainline; stay on the stable even minors",
-      "matchPackageNames": [
-        "nginx",
-        "docker.io/library/nginx"
-      ],
-      "allowedVersions": "/^1\\.\\d*[02468](\\.|-|$)/"
-    },
     {
       "description": "Group the patch and minor bumps of a repo's GitHub/Forgejo actions",
       "matchManagers": [