Summary

This change reverts the ability to display a UTC timestamp with a custom strftime format string in TimeStamper. Previously, the local time zone was used regardless of utc=True flag for TimeStamper. The output for utc=False remains unchanged, and timezone is correctly displayed with %z and %Z.

from structlog.processors import TimeStamper
ts = TimeStamper(fmt="%Y-%m-%d %H:%M:%S %z %Z")
print(ts(None, None, {}))

# BEFORE
{'timestamp': '2025-03-18 10:24:40 +0100 CET'}
# AFTER
{'timestamp': '2025-03-18 09:23:36 +0000 UTC'}

Due to limitations in freezegun, the tests don't simulate execution is different timezones. Maybe a switch to time-machine can fix that.

Feel free to remove comments and descriptions in changelog.

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