chore: remove leftover Scala 2.12 idioms

src/main/scala/org/apache/pekko/persistence/dynamodb/journal/DynamoDBJournal.scala

@@ -25,7 +25,6 @@ import org.apache.pekko.stream.{ Materializer, SystemMaterializer }
 import com.amazonaws.services.dynamodbv2.model._
 import com.typesafe.config.Config
 
-import scala.collection.immutable
 import scala.concurrent.{ ExecutionContext, Future, Promise }
 import scala.util.{ Success, Try }
 
@@ -107,7 +106,7 @@ class DynamoDBJournal(config: Config)
   private case class OpFinished(pid: String, f: Future[Done])
   private val opQueue: JMap[String, Future[Done]] = new JHMap
 
-  override def asyncWriteMessages(messages: immutable.Seq[AtomicWrite]): Future[immutable.Seq[Try[Unit]]] = {
+  override def asyncWriteMessages(messages: Seq[AtomicWrite]): Future[Seq[Try[Unit]]] = {
     val p = Promise[Done]()
     val pid = messages.head.persistenceId
     opQueue.put(pid, p.future)

src/main/scala/org/apache/pekko/persistence/dynamodb/journal/DynamoDBRecovery.scala

@@ -60,7 +60,7 @@ object DynamoDBRecovery {
  * @param partitionSeqNum - the partition sequence number for the given persistence id.
  * @param partitionEventNums - will be 0-99, representing the event ordering within the given partition sequence.
  */
-case class PartitionKeys(partitionSeqNum: Long, partitionEventNums: immutable.Seq[Long])
+case class PartitionKeys(partitionSeqNum: Long, partitionEventNums: Seq[Long])
 
 /**
  * Groups Longs from a stream into a [PartitionKeys] whereas each sequence shall contain the values that would be within the

src/test/scala/org/apache/pekko/persistence/dynamodb/journal/DynamoPartitionGroupedSpec.scala

@@ -50,7 +50,7 @@ class DynamoPartitionGroupedSpec extends TestKit(ActorSystem("DynamoPartitionGro
         .request(1)
         .expectNext(PartitionKeys(2L, 200L to 299))
         .request(1)
-        .expectNext(PartitionKeys(3L, scala.collection.immutable.Seq(300L)))
+        .expectNext(PartitionKeys(3L, Seq(300L)))
         .expectComplete()
     }