[SPARK-37026][ML][BUILD] Ensure the element type of ResolvedRFormula.terms is scala.Seq for Scala 2.13

[sarutak]

What changes were proposed in this pull request?

This PR fixes the issue that scala.Seq[scala.collection.mutable.ArraySeq$ofRef] will be passed to ResolvedRFormula.terms though it expects scala.Seq[scala.Seq[String]] with Scala 2.13.
As of Scala 2.13, scala.Seq is scala.collection.immutable.Seq, so this issue happens.

Why are the changes needed?

Bug fix.
Due to this issue, ResolvedRFormula.toString throws ClassCastException.

java.lang.ClassCastException: scala.collection.mutable.ArraySeq$ofRef cannot be cast to scala.collection.immutable.Seq
        at scala.collection.immutable.List.map(List.scala:246)
        at scala.collection.immutable.List.map(List.scala:79)
        at org.apache.spark.ml.feature.ResolvedRFormula.toString(RFormulaParser.scala:143)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:244)
        at py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:357)
        at py4j.Gateway.invoke(Gateway.java:282)
        at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
        at py4j.commands.CallCommand.execute(CallCommand.java:79)
        at py4j.ClientServerConnection.waitForCommands(ClientServerConnection.java:182)
        at py4j.ClientServerConnection.run(ClientServerConnection.java:106)
        at java.lang.Thread.run(Thread.java:748)

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New test is added and build/sbt -Pscala-2.13 "testOnly org.apache.spark.ml.feature.RFormulaSuite" passes.
CIs should ensure that this change works with Scala 2.12 too.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48806/

[SparkQA]

Test build #144327 has finished for PR 34301 at commit 76e499e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/48806/

[HyukjinKwon]

Merged to master.