ZTS: fix history_007_pos test on Ubuntu 24.04 (#16410)

The timezone "US/Mountain" isn't supported on newer linux versions.
Using the correct timezone "America/Denver" like it's done in FreeBSD
will fix this. Older Linux distros should behave also okay with this.

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>

tests/zfs-tests/tests/functional/history/history.cfg

@@ -37,11 +37,7 @@ export TMP_HISTORY=$TEST_BASE_DIR/tmp_history.$$
 export NEW_HISTORY=$TEST_BASE_DIR/new_history.$$
 
 export MIGRATEDPOOLNAME=${MIGRATEDPOOLNAME:-history_pool}
-if is_freebsd; then
-	export TIMEZONE=${TIMEZONE:-America/Denver}
-else
-	export TIMEZONE=${TIMEZONE:-US/Mountain}
-fi
+export TIMEZONE=${TIMEZONE:-America/Denver}
 
 export HIST_USER="huser"
 export HIST_GROUP="hgroup"