Remove cron trigger implementation

- Remove cron-trigger.ts and cron-scheduler.ts files
- Clean up cron-related code from plugin index and workflow hooks
- Remove cron references from workflow executor types
- Add cron to NOT-IMPLEMENTING.md with webhook alternatives
- Update README with scheduled workflow documentation using external services
- Suggest GitHub Actions and Vercel Cron as reliable alternatives

Benefits of external scheduling:
- Better reliability and process isolation
- Easier debugging and monitoring
- Leverages existing cloud infrastructure
- Reduces plugin complexity and maintenance burden

src/core/workflow-executor.ts

@@ -13,8 +13,6 @@ export type PayloadWorkflow = {
       collectionSlug?: string | null
       operation?: string | null
       webhookPath?: string | null
-      cronExpression?: string | null
-      timezone?: string | null
       global?: string | null
       globalOperation?: string | null
       [key: string]: unknown