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

README.md

@@ -10,7 +10,7 @@ one PR per version bump, with release notes.
   to start getting bumps.
 - **Shared rules:** `default.json`, used as `local>asxpi/renovate`:
   `deps:` commit prefix, no dependency dashboard, bundler pinned to
-  2.7.2 (nixpkgs' version), Postgres majors flagged as migrations,
+
   action bumps grouped, Renovate itself monthly.
 - **Secrets:** `RENOVATE_TOKEN` (renovate-bot's token) and
   `GH_COM_TOKEN` (read-only github.com token for release notes).