Bazel: Add feature_flags_v2 to RABBITMQ_FEATURE_FLAGS

This is required for mixed-version testing where all nodes need to start
with the required feature flags enabled.

Author: dumbbell

rabbitmq.bzl

@@ -272,7 +272,9 @@ def rabbitmq_integration_suite(
             #   virtual_host_metadata
             #   maintenance_mode_status
             #   user_limits
-            "RABBITMQ_FEATURE_FLAGS": "quorum_queue,implicit_default_bindings,virtual_host_metadata,maintenance_mode_status,user_limits",
+            # Starting from 3.12.0:
+            #   feature_flags_v2
+            "RABBITMQ_FEATURE_FLAGS": "quorum_queue,implicit_default_bindings,virtual_host_metadata,maintenance_mode_status,user_limits,feature_flags_v2",
             "RABBITMQ_RUN": "$TEST_SRCDIR/$TEST_WORKSPACE/{}/rabbitmq-for-tests-run".format(package),
             "RABBITMQCTL": "$TEST_SRCDIR/$TEST_WORKSPACE/{}/broker-for-tests-home/sbin/rabbitmqctl".format(package),
             "RABBITMQ_PLUGINS": "$TEST_SRCDIR/$TEST_WORKSPACE/{}/broker-for-tests-home/sbin/rabbitmq-plugins".format(package),