use correct timezone on bookworm imaging

US/Eastern works on bookworm, but America/Detroit works on every
version. Remove a needless difference between bookworm and trixie
preseeds.

Author: rrotter

templates/virtual_machine/bookworm.cfg.erb

@@ -152,7 +152,7 @@ d-i clock-setup/utc boolean true
 
 # You may set this to any valid setting for $TZ; see the contents of
 # /usr/share/zoneinfo/ for valid values.
-d-i time/zone string US/Eastern
+d-i time/zone string America/Detroit
 
 # Controls whether to use NTP to set the clock during the install
 d-i clock-setup/ntp boolean true