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

[tmdeveloper007]

Summary of What Has Been Done

Replaced all occurrences of the deprecated datetime.utcnow() with datetime.now(timezone.utc) in src/utils/code_review.py.

Changes Made

• Changed from datetime import datetime to from datetime import datetime, timezone in the import statement
• Replaced datetime.utcnow() with datetime.now(timezone.utc) in 5 locations across the following methods:
  • submit_code method (submitted_at timestamp)
  • start_review method (review_id generation and started_at timestamp)
  • add_feedback_comment method (created_at timestamp)
  • complete_review method (completed_at timestamp)

Impact it Made

• Eliminates Python 3.12+ deprecation warnings for datetime.utcnow()
• Uses the recommended timezone-aware datetime approach throughout the code review module
• Correct timestamp generation for code submissions, reviews, and feedback

Closes #1407.

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[komalharshita]

@tmdeveloper007 kindly resolve the merge conflicts