Fix some hardcoded scala versions (#2768)

The Scala version used in `MillBuildBootstrapModule` to compile the
`build.sc` was hardcoded to `2.13.10`. Instead, we want to always use
the same version we used to build that Mill version.

As an side effect, Mill is now compatible with new Java 21, due to the
use of a newer Scala version in the bootstrap process.

Pull request: #2768

build.sc

@@ -521,14 +521,14 @@ object main extends MillStableScalaModule with BuildInfo {
       trait CaseModule extends ScalaModule with Cross.Module[String] {
         def caseName = crossValue
         object external extends ScalaModule {
-          def scalaVersion = "2.13.10"
+          def scalaVersion = Deps.scalaVersion
         }
 
         def moduleDeps = Seq(external)
 
         val Array(prefix, suffix, rest) = caseName.split("-", 3)
         def millSourcePath = super.millSourcePath / prefix / suffix / rest
-        def scalaVersion = "2.13.10"
+        def scalaVersion = Deps.scalaVersion
         def ivyDeps = T {
           if (!caseName.contains("realistic") && !caseName.contains("sourcecode")) super.ivyDeps()
           else Agg(

integration/feature/watch-source-input/test/src/WatchSourceInputTests.scala

@@ -47,6 +47,7 @@ object WatchSourceInputTests extends IntegrationTestSuite {
       val res = f(expectedOut, expectedErr, expectedShows)
       val (shows, out) = res.out.linesIterator.toVector.partition(_.startsWith("\""))
       val err = res.err.linesIterator.toVector
+        .filter(!_.contains("Compiling compiler interface..."))
         .filter(!_.contains("Watching for changes"))
         .filter(!_.contains("[info] compiling"))
         .filter(!_.contains("[info] done compiling"))

runner/src/mill/runner/MillBuildRootModule.scala

@@ -4,7 +4,7 @@ import coursier.Repository
 import mill._
 import mill.api.{PathRef, Result, internal}
 import mill.define.{Discover, Task}
-import mill.scalalib.{BoundDep, Dep, DepSyntax, Lib, ScalaModule}
+import mill.scalalib.{Dep, DepSyntax, Lib, ScalaModule}
 import mill.util.CoursierSupport
 import mill.util.Util.millProjectModule
 import mill.scalalib.api.Versions
@@ -41,7 +41,7 @@ class MillBuildRootModule()(implicit
   override def millSourcePath = millBuildRootModuleInfo.projectRoot / os.up / "mill-build"
   override def intellijModulePath: os.Path = millSourcePath / os.up
 
-  override def scalaVersion: T[String] = "2.13.10"
+  override def scalaVersion: T[String] = BuildInfo.scalaVersion
 
   /**
    * All script files (that will get wrapped later)