Fix proxy caching config additions in terms of HSTS

See tootsuite/mastodon#8487

Running-Mastodon/Tuning.md

@@ -158,6 +158,7 @@ index 0b50542..1d3fac6 100644
 +    proxy_cache_valid 200 7d;
 +    proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
 +    add_header X-Cached $upstream_cache_status;
++    add_header Strict-Transport-Security "max-age=31536000";
 +
      tcp_nodelay on;
    }