kafka/server: run activate_feature in background

Ufter upgrading node in cluster to 22.1.x version from 22.11.x
upgraded node isn't controller leade, it enters group_metadata_migration::start
and hits the "kafka_internal/group topic does not exists, activating" path
this call waits for activate_feature,activate_feature loops until the feature is active,
but it cannot be activated because only the controller leader runs the feature_manager
logic for activating features, and the controller leader is a 21.11.x node
that doesn't have the code. The node remains in 'booting' state indefinitely

Fixes: redpanda-data#4469

src/v/kafka/server/group_metadata_migration.cc

@@ -687,11 +687,10 @@ ss::future<> group_metadata_migration::start(ss::abort_source& as) {
           mlog.info,
           "kafka_internal/group topic does not exists, activating "
           "consumer_offsets feature");
-        try {
-            co_await activate_feature(as);
-        } catch (const ss::abort_requested_exception&) {
-            // ignore abort requested exception, we are shutting down
-        }
+
+        ssx::spawn_with_gate(_background_gate, [this, &as]() -> ss::future<> {
+            return activate_feature(as);
+        });
         co_return;
     }
     // otherwise wait for feature to be preparing and execute migration