Fix TraversableOnce deprecation warnings

Fixes following warning:
type TraversableOnce in package scala is deprecated (since 2.13.0): Use IterableOnce instead of TraversableOnce

Change-Id: I641588ee14ea4c52000959eebf6714ae3338b1fd

build.sbt

diff --git a/build.sbt b/build.sbt
index d24135d..144f784 100644
--- a/build.sbt
+++ b/build.sbt
@@ -8,7 +8,7 @@
 
 concurrentRestrictions in Global += Tags.limit(Tags.Test, 1)
 
-ThisBuild / scalacOptions += "-feature"
+ThisBuild / scalacOptions ++= Seq("-deprecation", "-feature")
 
 lazy val root = (project in file("."))
   .settings(

src/main/scala/com/googlesource/gerrit/plugins/analytics/Contributors.scala

diff --git a/src/main/scala/com/googlesource/gerrit/plugins/analytics/Contributors.scala b/src/main/scala/com/googlesource/gerrit/plugins/analytics/Contributors.scala
index b9427d3..5047805 100644
--- a/src/main/scala/com/googlesource/gerrit/plugins/analytics/Contributors.scala
+++ b/src/main/scala/com/googlesource/gerrit/plugins/analytics/Contributors.scala
@@ -139,7 +139,7 @@
 
   def get(projectRes: ProjectResource, startDate: Option[Long], stopDate: Option[Long],
           aggregationStrategy: AggregationStrategy, extractBranches: Boolean)
-  : TraversableOnce[UserActivitySummary] = {
+  : IterableOnce[UserActivitySummary] = {
 
     Using.resource(repoManager.openRepository(projectRes.getNameKey)) { repo =>
       val stats = new Statistics(projectRes.getNameKey, commitsStatisticsCache)

src/main/scala/com/googlesource/gerrit/plugins/analytics/common/GsonFormatter.scala

diff --git a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/GsonFormatter.scala b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/GsonFormatter.scala
index 310ccd0..70df461 100644
--- a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/GsonFormatter.scala
+++ b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/GsonFormatter.scala
@@ -28,7 +28,7 @@
       .registerTypeHierarchyAdapter(classOf[Iterable[Any]], new IterableSerializer)
       .registerTypeHierarchyAdapter(classOf[Option[Any]], new OptionSerializer())
 
-  def format[T](values: TraversableOnce[T], out: PrintWriter) = {
+  def format[T](values: IterableOnce[T], out: PrintWriter) = {
     val gson: Gson = gsonBuilder.create
 
     for (value <- values) {

src/main/scala/com/googlesource/gerrit/plugins/analytics/common/JsonStreamedResultBuilder.scala

diff --git a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/JsonStreamedResultBuilder.scala b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/JsonStreamedResultBuilder.scala
index de01b79..7e3c507 100644
--- a/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/JsonStreamedResultBuilder.scala
+++ b/src/main/scala/com/googlesource/gerrit/plugins/analytics/common/JsonStreamedResultBuilder.scala
@@ -21,7 +21,7 @@
 import scala.util.Using
 
 class GsonStreamedResult[T](val jsonFmt: GsonFormatter,
-                            val committers: TraversableOnce[T]) extends BinaryResult {
+                            val committers: IterableOnce[T]) extends BinaryResult {
   override def writeTo(os: OutputStream) =
     Using.resource(new PrintWriter(os))(jsonFmt.format(committers, _))
 }