[ENH] Replace deprecated utcnow() invocations

[peytondmurray]

Fixes #1025.

Description

This PR replaces all calls to datetime.datetime.utcnow() with datetime.datetime.now(tz=datetime.UTC) to avoid deprecation warnings.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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[soapy1]

It looks like this won't work for python 3.10. From the test logs:

conda-store-worker-1  | [2025-01-27 20:27:50,453: ERROR/ForkPoolWorker-4] Task task_update_conda_channel[6a768418-841e-4606-8ea2-107a61e6e34b] raised unexpected: AttributeError("module 'datetime' has no attribute 'UTC'")
conda-store-worker-1  | Traceback (most recent call last):
conda-store-worker-1  |   File "/opt/conda/envs/conda-store-server/lib/python3.10/site-packages/celery/app/trace.py", line 453, in trace_task
conda-store-worker-1  |     R = retval = fun(*args, **kwargs)
conda-store-worker-1  |   File "/opt/conda/envs/conda-store-server/lib/python3.10/site-packages/celery/app/trace.py", line 736, in __protected_call__
conda-store-worker-1  |     return self.run(*args, **kwargs)
conda-store-worker-1  |   File "/opt/conda-store-server/conda_store_server/_internal/worker/tasks.py", line 190, in task_update_conda_channel
conda-store-worker-1  |     channel.update_packages(db, subdirs=settings.conda_platforms)
conda-store-worker-1  |   File "/opt/conda-store-server/conda_store_server/_internal/orm.py", line 719, in update_packages
conda-store-worker-1  |     self.last_update = datetime.datetime.now(tz=datetime.UTC)
conda-store-worker-1  | AttributeError: module 'datetime' has no attribute 'UTC'

[soapy1]

🎠