chore: make metals believe scala-library-bootstrapped is a scala 3 project (#23568)

If we don't set explicitly the scala version, sbt will default to `2.12`
which will be then used by metals, even when we actually override the
scalaInstance when bootstrapping. In this PR, we actually make sure that
the stdlib bootstrapped uses a Scala 3 version so that metals won't
confuse it with a scala 2.12 project.

<img width="1200" height="42" alt="Screenshot 2025-07-18 at 20 45 30"
src="https://github.com/user-attachments/assets/bc1a66c0-48c3-4dea-9d64-3f651170ef72"
/>

Author: hamzaremmal

project/Build.scala

@@ -1450,6 +1450,9 @@ object Build {
       moduleName    := "scala-library",
       version       := dottyVersion,
       versionScheme := Some("semver-spec"),
+      // sbt defaults to scala 2.12.x and metals will report issues as it doesn't consider the project a scala 3 project 
+      // (not the actual version we use to compile the project)
+      scalaVersion  := referenceVersion,
       crossPaths    := false, // org.scala-lang:scala-library doesn't have a crosspath
       // Add the source directories for the stdlib (non-boostrapped)
       Compile / unmanagedSourceDirectories := Seq(baseDirectory.value / "src"),