Release 3.1.5: remove debug println, drop worksheet leftover, modernize CI

- Remove stray println(child.value) from TreeNode.removeChild()
- Add regression test for removeChild() return value
- Delete leftover ExampleUnitTest.kt template test
- Remove Example.ws.kts worksheet and kotlin("script-runtime") from jvmMain
- Bump actions/checkout v2 -> v4 in CI workflows
- Update README install snippets to 3.1.5
- Bump version to 3.1.5

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNode.kt

@@ -62,7 +62,6 @@ open class TreeNode<T>(val value: T, var treeIterator: TreeNodeIterators = PreOr
      * @return `true` if the node has been successfully removed; `false` if it was not present in the tree.
      */
     fun removeChild(child: TreeNode<T>): Boolean {
-        println(child.value)
         val removed = child._parent?._children?.remove(child)
         child._parent = null
         return removed ?: false