Refactor MiMaFilters

Extract the Scala 2 library bootstrapped breaking changes into a separate
file. These are not that important, there we care more about the TASTy MiMa.
This makes it simpler to read and maintain the MiMaFilters.

Also some renaming to make the semantics of the lists more explicit.

project/Build.scala

@@ -1116,9 +1116,9 @@ object Build {
         _.filterNot(file => file.data.getName == s"scala-library-$stdlibBootstrappedVersion.jar")
       },
       mimaCheckDirection := "both",
-      mimaBackwardIssueFilters := MiMaFilters.StdlibBootstrappedBackwards,
-      mimaForwardIssueFilters := MiMaFilters.StdlibBootstrappedForward,
-      customMimaReportBinaryIssues("MiMaFilters.{StdlibBootstrappedForward, StdlibBootstrappedBackwards}"),
+      mimaBackwardIssueFilters := Scala2LibraryBootstrappedMiMaFilters.BackwardsBreakingChanges,
+      mimaForwardIssueFilters := Scala2LibraryBootstrappedMiMaFilters.ForwardsBreakingChanges,
+      customMimaReportBinaryIssues("Scala2LibraryBootstrappedMiMaFilters.{BackwardsBreakingChanges, ForwardsBreakingChanges}"),
       mimaPreviousArtifacts += "org.scala-lang" % "scala-library" % stdlibBootstrappedVersion,
       mimaExcludeAnnotations ++= Seq(
         "scala.annotation.experimental",
@@ -2177,9 +2177,9 @@ object Build {
             }
             (thisProjectID.organization % crossedName % ltsDottyVersion)
           },
-          mimaBackwardIssueFilters := MiMaFilters.LibraryBackwards,
-          mimaForwardIssueFilters := MiMaFilters.LibraryForward,
-          customMimaReportBinaryIssues("MiMaFilters.{LibraryForward, LibraryBackwards}"),
+          mimaForwardIssueFilters := MiMaFilters.Scala3Library.ForwardsBreakingChanges,
+          mimaBackwardIssueFilters := MiMaFilters.Scala3Library.BackwardsBreakingChanges,
+          customMimaReportBinaryIssues("MiMaFilters.Scala3Library.{ForwardsBreakingChanges, BackwardsBreakingChanges}"),
         )
       } else base
     }