Scala 2.13 version deprecation warning

[cosmir17]

Scala 2.13 version deprecation warning

Expected behavior

No Warning messages

Actual behavior

[warn] /Users/.../my-project/target/scala-2.13/src_managed/main/thrift/...../thriftscala/MyScalaClass.scala:530:10: method flatMap in class IterableOnceExtensionMethods is deprecated (since 2.13.0): Use .iterator.flatMap instead or consider requiring an Iterable
[warn]     (ids.flatMap { id => getFieldBlob(id).map { fieldBlob => (id, fieldBlob) } }).toMap

Steps to reproduce the behavior

Scrooge dependency 20.5.0 and Scala 2.13 build.

[yufangong]

Hi @cosmir17, thanks for the report. Are you interested in taking a stab at this?
I think this piece sits in here. And some gold file templates updates afterward. Thanks.