fix: 修改工单迁移文件warining问题

Any fields that are stored with VARCHAR column types may have their max_length restricted to 255 characters if you are using unique=True for the field.
See: https://docs.djangoproject.com/en/3.1/ref/databases/#mysql-character-fields

apps/tickets/migrations/0013_ticket_serial_num.py

@@ -38,7 +38,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='ticket',
             name='serial_num',
-            field=models.CharField(max_length=256, null=True, unique=True, verbose_name='Serial number'),
+            field=models.CharField(max_length=128, null=True, unique=True, verbose_name='Serial number'),
         ),
         migrations.RunPython(fill_ticket_serial_number),
     ]

apps/tickets/models/ticket.py

@@ -77,7 +77,7 @@ class Ticket(CommonModelMixin, OrgModelMixin):
         'TicketFlow', related_name='tickets', on_delete=models.SET_NULL, null=True,
         verbose_name=_("TicketFlow")
     )
-    serial_num = models.CharField(max_length=256, unique=True, null=True, verbose_name=_('Serial number'))
+    serial_num = models.CharField(max_length=128, unique=True, null=True, verbose_name=_('Serial number'))
 
     class Meta:
         ordering = ('-date_created',)