Make CrossVersion.Disabled stable

Fixes https://github.com/sbt/sbt/issues/4975

This makes `CrossVersion.Disabled` a stable identifier by reverting `final def` back to `final val`.

This is to fix Scala.JS build

```
[error] ScalaJSCrossVersion.scala:34:23: stable identifier required, but sbt.`package`.CrossVersion.Disabled found.
[error]     case CrossVersion.Disabled =>
[error]                       ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
```

### notes

- #121 added `final val Disabled = sbt.librarymanagement.Disabled` but it was just a companion object
- #280 actually made it `final val Disabled = sbt.librarymanagement.Disabled()`, but this broke Cat's build that was calling `CrossVersion.Disabled()`
- #290 changed to `final def Disabled = sbt.librarymanagement.Disabled` and `object Disabled extends sbt.librarymanagement.Disabled`
- This changes back to `final val Disabled = sbt.librarymanagement.Disabled` (but because we changed the companion object in #290 that's ok)

core/src/main/scala/sbt/librarymanagement/CrossVersion.scala

@@ -39,7 +39,7 @@ sealed class Disabled private () extends sbt.librarymanagement.CrossVersion() wi
 
 }
 object Disabled extends sbt.librarymanagement.Disabled {
-  def apply(): Disabled = new Disabled()
+  def apply(): Disabled = Disabled
 }
 
 /**

core/src/main/scala/sbt/librarymanagement/CrossVersionExtra.scala

@@ -8,7 +8,7 @@ final case class ScalaVersion(full: String, binary: String)
 private[librarymanagement] abstract class CrossVersionFunctions {
 
   /** Compatibility with 0.13 */
-  final def Disabled = sbt.librarymanagement.Disabled
+  final val Disabled = sbt.librarymanagement.Disabled
   final val Binary = sbt.librarymanagement.Binary
   final val Constant = sbt.librarymanagement.Constant
   final val Full = sbt.librarymanagement.Full

core/src/test/scala/example/tests/CrossVersionCompatTest.scala

@@ -0,0 +1,70 @@
+package example.tests
+
+import sbt.librarymanagement.{ CrossVersion, Disabled }
+import verify.BasicTestSuite
+
+object CrossVersionCompatTest extends BasicTestSuite {
+  test("CrossVersion.Disabled is typed to be Disabled") {
+    assert(CrossVersion.Disabled match {
+      case _: Disabled => true
+      case _           => false
+    })
+  }
+
+  test("CrossVersion.Disabled functions as disabled") {
+    assert(CrossVersion(CrossVersion.disabled, "1.0.0", "1.0") == None)
+    assert(CrossVersion(CrossVersion.Disabled, "1.0.0", "1.0") == None)
+  }
+
+  test("CrossVersion.Disabled() is typed to be Disabled") {
+    assert(CrossVersion.Disabled() match {
+      case _: Disabled => true
+      case _           => false
+    })
+  }
+
+  test("CrossVersion.Disabled() functions as disabled") {
+    assert(CrossVersion(CrossVersion.disabled, "1.0.0", "1.0") == None)
+    assert(CrossVersion(CrossVersion.Disabled(), "1.0.0", "1.0") == None)
+  }
+
+  test("CrossVersion.Disabled is stable") {
+    assert(CrossVersion.Disabled match {
+      case CrossVersion.Disabled => true
+      case _                     => false
+    })
+  }
+
+  test("sbt.librarymanagement.Disabled is typed to be Disabled") {
+    assert(Disabled match {
+      case _: Disabled => true
+      case _           => false
+    })
+  }
+
+  test("sbt.librarymanagement.Disabled is stable") {
+    assert(Disabled match {
+      case Disabled => true
+      case _        => false
+    })
+  }
+
+  test("sbt.librarymanagement.Disabled() is typed to be Disabled") {
+    assert(Disabled() match {
+      case _: Disabled => true
+      case _           => false
+    })
+  }
+
+  test("CrossVersion.disabled is sbt.librarymanagement.Disabled") {
+    assert(CrossVersion.disabled == Disabled)
+  }
+
+  test("CrossVersion.Disabled is sbt.librarymanagement.Disabled") {
+    assert(CrossVersion.Disabled == Disabled)
+  }
+
+  test("CrossVersion.Disabled() is sbt.librarymanagement.Disabled") {
+    assert(CrossVersion.Disabled() == Disabled)
+  }
+}