Revert "Update to Drupal 9"

Revert back so that we can continue to use the Config Installer module.

This reverts commit e5b86c7f39.

composer.json

@@ -17,12 +17,13 @@
     "require": {
         "composer/installers": "^1.2",
         "drupal/admin_toolbar": "^2.0",
-        "drupal/core-composer-scaffold": "^9.0",
-        "drupal/core-recommended": "^9.0",
-        "drush/drush": "^10"
+        "drupal/config_installer": "^1.8",
+        "drupal/core-composer-scaffold": "^8.8",
+        "drupal/core-recommended": "^8.8",
+        "drush/drush": "^9"
     },
     "require-dev": {
-        "drupal/core-dev": "^9.0"
+        "drupal/core-dev": "^8.8"
     },
     "conflict": {
         "drupal/drupal": "*"