refactor: use renamed timezone utils

https://github.com/frappe/frappe/pull/20253
diff --git a/erpnext/www/book_appointment/index.py b/erpnext/www/book_appointment/index.py
index dfca946..f50c207 100644
--- a/erpnext/www/book_appointment/index.py
+++ b/erpnext/www/book_appointment/index.py
@@ -4,6 +4,7 @@
 import frappe
 import pytz
 from frappe import _
+from frappe.utils.data import get_system_timezone
 
 WEEKDAYS = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
 
@@ -125,7 +126,7 @@
 
 def convert_to_guest_timezone(guest_tz, datetimeobject):
 	guest_tz = pytz.timezone(guest_tz)
-	local_timezone = pytz.timezone(frappe.utils.get_time_zone())
+	local_timezone = pytz.timezone(get_system_timezone())
 	datetimeobject = local_timezone.localize(datetimeobject)
 	datetimeobject = datetimeobject.astimezone(guest_tz)
 	return datetimeobject
@@ -134,7 +135,7 @@
 def convert_to_system_timezone(guest_tz, datetimeobject):
 	guest_tz = pytz.timezone(guest_tz)
 	datetimeobject = guest_tz.localize(datetimeobject)
-	system_tz = pytz.timezone(frappe.utils.get_time_zone())
+	system_tz = pytz.timezone(get_system_timezone())
 	datetimeobject = datetimeobject.astimezone(system_tz)
 	return datetimeobject