fix : replaced deprecated datetime.utcnow with timezone-aware alternative in certification.py

[tmdeveloper007]

Summary of What Has Been Done

Replaced all occurrences of the deprecated datetime.utcnow() with datetime.now(timezone.utc) in src/utils/certification.py. This addresses the Python 3.12+ deprecation warning for naive UTC datetime objects.

Changes Made

• Changed from datetime import datetime to from datetime import datetime, timezone in the import statement
• Replaced 5 occurrences of datetime.utcnow() with datetime.now(timezone.utc) across the following methods:
  • create_assessment
  • submit_assessment
  • _issue_certificate
  • _generate_verification_code
  • issue_digital_badge

Impact it Made

• Code is now compatible with Python 3.12+ and future Python versions
• Uses timezone-aware datetime objects which prevent subtle timezone-related bugs
• Follows Python best practices as recommended by the official deprecation notice

Closes #1350

Note: please assign this PR to the tmdeveloper007 account.

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