nixos/nextcloud: set X-Robots-Tag to "noindex, nofollow"

Upstream did so in nextcloud/server#36689 and
Nextcloud now complains that

    The "X-Robots-Tag" HTTP header is not set to "noindex, nofollow".
    This is a potential security or privacy risk, as it is recommended
    to adjust this setting accordingly.

nixos/modules/services/web-apps/nextcloud.nix

@@ -1112,7 +1112,7 @@ in {
           ${optionalString (cfg.nginx.recommendedHttpHeaders) ''
             add_header X-Content-Type-Options nosniff;
             add_header X-XSS-Protection "1; mode=block";
-            add_header X-Robots-Tag none;
+            add_header X-Robots-Tag "noindex, nofollow";
             add_header X-Download-Options noopen;
             add_header X-Permitted-Cross-Domain-Policies none;
             add_header X-Frame-Options sameorigin;