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

[tmdeveloper007]

Summary of What Has Been Done

Replaced all 5 occurrences of the deprecated datetime.utcnow() with the timezone-aware datetime.now(timezone.utc) in src/utils/code_review.py.

Changes:

• Updated import: from datetime import datetime -> from datetime import datetime, timezone
• Updated submit_code: datetime.utcnow() -> datetime.now(timezone.utc)
• Updated start_review: datetime.utcnow().timestamp() -> datetime.now(timezone.utc).timestamp()
• Updated start_review started_at: datetime.utcnow() -> datetime.now(timezone.utc)
• Updated add_feedback_comment: datetime.utcnow() -> datetime.now(timezone.utc)
• Updated complete_review: datetime.utcnow() -> datetime.now(timezone.utc)

Changes Made

• 1 file changed, 6 insertions, 6 deletions

Impact it Made

• Eliminates DeprecationWarning in Python 3.12+
• All timestamps are consistently timezone-aware

Closes #1479

Note

Please assign this PR to the tmdeveloper007 account.

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