tests/rptest: acknowledge iceberg_extended_mode_config feature gate

A new feature, iceberg_extended_mode_config, was added gated at v26_2_1,
so it joins the set the v26.1 -> v26.2 perturbation coverage guard
reports as unavailable while the upgrade is unfinalized. That guard
fails unless every gated feature is exercised or acknowledged.

The feature gates Iceberg extended-mode topic-config validation;
perturbing it would need Iceberg topic setup orthogonal to the
finalization behavior under test, so acknowledge it rather than exercise
it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dotnwat

tests/rptest/tests/cluster_features_test.py

@@ -1211,6 +1211,9 @@ def test_get_upgrade_status_reports_in_progress(self):
         # Cluster-linking features, out of scope for this single-cluster test.
         "shadow_link_sr_api_sync",
         "batch_mirror_topic_status",
+        # Iceberg extended-mode topic-config gate; exercising it needs Iceberg
+        # topic setup orthogonal to the finalization behavior under test.
+        "iceberg_extended_mode_config",
     }
 )