[SPARK-32971][K8S][FOLLOWUP] Add .toSeq for Scala 2.13 compilation

### What changes were proposed in this pull request?

This is a follow-up to fix Scala 2.13 compilation at Kubernetes module.

### Why are the changes needed?

To fix Scala 2.13 compilation.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the GitHub Action Scala 2.13 compilation job.

Closes #29859 from dongjoon-hyun/SPARK-32971-2.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: dongjoon-hyun

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/features/MountVolumesFeatureStep.scala

@@ -114,7 +114,7 @@ private[spark] class MountVolumesFeatureStep(conf: KubernetesConf)
   }
 
   override def getAdditionalKubernetesResources(): Seq[HasMetadata] = {
-    additionalResources
+    additionalResources.toSeq
   }
 }