Complete deprecation of install_ensure_config_directory()

+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -121,7 +121,7 @@ protected function prepareConfigDirectories() {
+    if (!file_prepare_directory($path, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS)) {

In install_ensure_config_directory
we were also doing:
file_prepare_directory($config_directory, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);

So that means that functionally we're doing the same thing, looks very good.

Confirmed that this is the last reference:

[ibnsina:maintainer | Fri 16:01:33] $ grep -r "install_ensure_config_directory" *
core/includes/install.inc:function install_ensure_config_directory($type) {
core/modules/simpletest/src/KernelTestBase.php:    if (!install_ensure_config_directory(CONFIG_SYNC_DIRECTORY)) {

The change record in https://www.drupal.org/node/2501187 doesn't mention this function and only refers to the long-gone active directory. It looks like the CR reference was added in #2873750: Add Change record to @deprecated for install_ensure_config_directory(), but let's either update the change record appropriately or switch to a better one if there's a better one?

Committed and pushed to 8.5.x and cherry-picked to 8.4.x. Thanks!