Revert "Drop support for Drupal 8."

This reverts commit 63f89f12.

composer.json

@@ -6,9 +6,9 @@
     "require": {
         "drupal/autosave_form": "^1.2",
         "drupal/conflict": "^2.0-alpha2",
-        "drupal/core": "^9.0.6",
+        "drupal/core": "^8.8.10 || ^9.0.6",
         "drupal/diff": "^1.0",
-        "drupal/lightning_core": "5.*",
+        "drupal/lightning_core": "3.* || 4.* || 5.*",
         "drupal/moderation_dashboard": "^1.0",
         "drupal/moderation_sidebar": "^1.2"
     },

lightning_workflow.info.yml

 name: 'Lightning Workflow'
 core: 8.x
-core_version_requirement: '^9'
+core_version_requirement: '^8 || ^9'
 type: module
 description: 'Tools to improve your content workflow.'
 package: Lightning

modules/lightning_scheduler/lightning_scheduler.info.yml

 name: Scheduler
 core: 8.x
-core_version_requirement: '^9'
+core_version_requirement: '^8 || ^9'
 type: module
 description: 'Allows you to schedule changes in moderation state.'
 package: Lightning