Remove hardcoded lf versions: `ExternalCallIT`: don't hardcode required dam-test dar

Right now, we have

  override lazy val darPath: Path = rlocation(
    Paths.get("daml-script/test/external-call-test-v2.4.dar")
  )

This is part of a bigger ticket, we need to determine a princpiled way to solve these versions. In this case, the "highest stable version that supports external_call" seems like a good one

CC @angelol