Avoid deprecated utcnow()

[berland]

Issue
Resolves warning when running tests.

Approach
Do as suggested now that Python 3.8 has been dropped.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.72%. Comparing base (855c517) to head (143ab5c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9630      +/-   ##
==========================================
- Coverage   91.73%   91.72%   -0.01%     
==========================================
  Files         430      430              
  Lines       26639    26639              
==========================================
- Hits        24437    24435       -2     
- Misses       2202     2204       +2     

Flag	Coverage Δ
cli-tests	39.46% <ø> (ø)
everest-models-test	34.19% <50.00%> (-0.01%)	⬇️
gui-tests	71.83% <ø> (ø)
integration-test	38.29% <100.00%> (ø)
performance-tests	51.69% <ø> (+<0.01%)	⬆️
test	39.61% <100.00%> (ø)
unit-tests	73.99% <50.00%> (-0.01%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9630 will not alter performance

_{Comparing berland:utcnow (143ab5c) with main (855c517)}

Summary

✅ 24 untouched benchmarks

[andreas-el]

🏁