Replace all uses of UTC timezone with more standard Etc/UTC

This replaces all use of the UTC Olson timezone with the Etc/UTC
timezone. I'm uncertain which is more proper, but there are systems that
lack the UTC timezone, but I've not found any lacking the Etc/UTC
timezone. This resolves an issue where users were not able to update and
run even after importing their timezone database into MySQL.

There may still be some lingering instances I've missed, but those
should only affect systems that were already unable to function due to
lack of the UTC timezone.

Fixes #10906