Fix: remove invalid 'expired' value from proxy_cache_use_stale

proxy_cache_use_stale does not support 'expired'. Stale-on-expiry is
handled by 'updating' + proxy_cache_background_update on — nginx serves
the stale entry and revalidates in the background when proxy_cache_valid
time elapses.

Author: Robbie1977

nginx.conf.template

@@ -30,7 +30,7 @@ http {
     proxy_cache_valid 400 ${CACHE_STALE_TIME};
     proxy_cache_valid 404 600s;
 
-    proxy_cache_use_stale error timeout expired updating http_500 http_502 http_503 http_504 http_404;
+    proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504 http_404;
     proxy_cache_background_update on;
     proxy_cache_lock on;
     proxy_cache_lock_timeout 30s;