fix: use timezone-aware S3 partition timestamps

[Ghraven]

Summary

• replace deprecated datetime.utcnow() in the S3 tracking client partition timestamp
• use datetime.now(datetime.timezone.utc) so generated partition timestamps are timezone-aware

Before / after

Before, S3TrackingClient.save() generated a naive UTC timestamp with datetime.datetime.utcnow().isoformat().
After, it generates an explicit UTC timestamp such as 2026-05-24T03:12:34+00:00.

Verification

• python -m py_compile burr/tracking/s3client.py
• git diff --check

[github-actions]

This PR has had no activity for 96 days.
Closing to keep the PR list manageable.

Feel free to reopen when you are ready to continue.
If the branch has conflicts, a fresh PR against main might be easier.