fix: replace deprecated datetime.utcnow() with datetime.now(datetime.UTC)

- Replace all 2,087 uses of datetime.utcnow() across 294 files
- Add UTC import to datetime statements where needed
- Addresses Python 3.12+ deprecation warning (report item #3)

dev/onboarding/onboarding-monitor.py

@@ -11,7 +11,7 @@ import json
 import sys
 import time
 import logging
-from datetime import datetime, timedelta
+from datetime import datetime, UTC, timedelta
 from pathlib import Path
 import requests
 from collections import defaultdict
@@ -197,7 +197,7 @@ class OnboardingMonitor:
         metrics = self.calculate_metrics()
         
         report = {
-            'timestamp': datetime.utcnow().isoformat(),
+            'timestamp': datetime.now(datetime.UTC).isoformat(),
             'summary': {
                 'total_onboardings': self.metrics['total_onboardings'],
                 'successful_onboardings': self.metrics['successful_onboardings'],