Make ComponentSelectionWithCurrent.currentVersion not null

* ComponentSelectionWithCurrent objects are only created in one place and the currentVersion argument passed there is never null, so it's safe to make the property non-null as it effectively never is.
* This also fixes some issues in the Kotlin example, in which the dependencyUpdates task configuration was assumming the `currentVersion` property was not-null.

gradle-versions-plugin/src/main/kotlin/com/github/benmanes/gradle/versions/updates/resolutionstrategy/ComponentSelectionWithCurrent.kt

@@ -4,7 +4,7 @@ import org.gradle.api.artifacts.ComponentSelection
 
 class ComponentSelectionWithCurrent(
   private val delegate: ComponentSelection,
-  val currentVersion: String? = null,
+  val currentVersion: String,
 ) : ComponentSelection by delegate {
 
   override fun toString(): String {