chore: replace one last django.utils.timezone.utc with datetime.timezone.utc

[pxpeterxu]

Motivation

I was trying to deploy DMOJ to a Ubuntu 24.04 LTS server, and for it to work, I had to upgrade to the latest version of Django. I realized that 6285f97 already fixed most cases of django.utils.timezone.utc no longer existing (thanks @kiritofeng!), but there was one instance that was still causing errors.

Changes

• Replace contests.py use of timezone.utc with datetime.timezone.utc too

Testing

Ran:

python3 -Wa manage.py test

on a server with MySQL, etc. installed as recommended in https://docs.dmoj.ca/#/site/installation

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.73%. Comparing base (fd7fb05) to head (c1fc4d5).
⚠️ Report is 75 commits behind head on master.

Files with missing lines	Patch %	Lines
judge/views/contests.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## master #2423 +/- ## ========================================== + Coverage 46.76% 47.73% +0.96%  ========================================== Files 251 260 +9 Lines 13317 13746 +429 ========================================== + Hits 6228 6561 +333  - Misses 7089 7185 +96 

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