Remove @nowarn from Scala 2.12 build

Since it is shipped as part of scala-library since 2.12.3

Fixes #426

Author: raboof

build.sbt

@@ -89,7 +89,9 @@ lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform
         import com.typesafe.tools.mima.core._
         import com.typesafe.tools.mima.core.ProblemFilters._
         Seq(
-          exclude[ReversedMissingMethodProblem]("scala.collection.compat.PackageShared.*") // it's package-private
+          exclude[ReversedMissingMethodProblem]("scala.collection.compat.PackageShared.*"), // it's package-private
+          exclude[MissingClassProblem]("scala.annotation.nowarn"), // it's part of scala-library from 2.12.13 onwards
+          exclude[MissingClassProblem]("scala.annotation.nowarn$") // it's part of scala-library from 2.12.13 onwards
         )
       },
     )