Updated MiMa instructions in README.md to use latest scala versions.
Updated MiMa instructions in README.md to use latest scala versions.

scalatest/scalatest,dotty-staging/scalatest,cheeseng/scalatest,cheeseng/scalatest,scalatest/scalatest,cheeseng/scalatest,scalatest/scalatest,dotty-staging/scalatest,dotty-staging/scalatest