initialize_voyager_db: Use do_change_is_admin.

This is a code cleanup; in general, we avoid making direct updates to
model objects and then saving with a bare `.save()` across the
codebase.

zerver/management/commands/initialize_voyager_db.py

@@ -5,6 +5,7 @@ from typing import Any, Iterable, Tuple, Optional
 from django.conf import settings
 from django.core.management.base import BaseCommand
 
+from zerver.lib.actions import do_change_is_admin
 from zerver.lib.bulk_create import bulk_create_users
 from zerver.models import Realm, UserProfile, \
     email_to_username, get_client, get_system_bot
@@ -51,8 +52,7 @@ class Command(BaseCommand):
 
         # Initialize the email gateway bot as an API Super User
         email_gateway_bot = get_system_bot(settings.EMAIL_GATEWAY_BOT)
-        email_gateway_bot.is_api_super_user = True
-        email_gateway_bot.save()
+        do_change_is_admin(email_gateway_bot, True, permission="api_super_user")
 
         self.stdout.write("Successfully populated database with initial data.\n")
         self.stdout.write("Please run ./manage.py generate_realm_creation_link "