Bump version to 3.0.0-M15.
Bump version to 3.0.0-M15.

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