fix: update bin modified timestamp when updating qty

These timestamps are used for writing integrations hence whenever bin is
updated timestamp should update to reliabily use Bin for integration
logic.
diff --git a/erpnext/buying/doctype/purchase_order/test_purchase_order.py b/erpnext/buying/doctype/purchase_order/test_purchase_order.py
index 9a63afc..645e97e 100644
--- a/erpnext/buying/doctype/purchase_order/test_purchase_order.py
+++ b/erpnext/buying/doctype/purchase_order/test_purchase_order.py
@@ -682,17 +682,18 @@
 
 		bin1 = frappe.db.get_value("Bin",
 			filters={"warehouse": "_Test Warehouse - _TC", "item_code": "_Test Item"},
-			fieldname=["reserved_qty_for_sub_contract", "projected_qty"], as_dict=1)
+			fieldname=["reserved_qty_for_sub_contract", "projected_qty", "modified"], as_dict=1)
 
 		# Submit PO
 		po = create_purchase_order(item_code="_Test FG Item", is_subcontracted="Yes")
 
 		bin2 = frappe.db.get_value("Bin",
 			filters={"warehouse": "_Test Warehouse - _TC", "item_code": "_Test Item"},
-			fieldname=["reserved_qty_for_sub_contract", "projected_qty"], as_dict=1)
+			fieldname=["reserved_qty_for_sub_contract", "projected_qty", "modified"], as_dict=1)
 
 		self.assertEqual(bin2.reserved_qty_for_sub_contract, bin1.reserved_qty_for_sub_contract + 10)
 		self.assertEqual(bin2.projected_qty, bin1.projected_qty - 10)
+		self.assertNotEqual(bin1.modified, bin2.modified)
 
 		# Create stock transfer
 		rm_item = [{"item_code":"_Test FG Item","rm_item_code":"_Test Item","item_name":"_Test Item",
diff --git a/erpnext/stock/stock_balance.py b/erpnext/stock/stock_balance.py
index 10a14b9..62017e4 100644
--- a/erpnext/stock/stock_balance.py
+++ b/erpnext/stock/stock_balance.py
@@ -3,7 +3,7 @@
 
 
 import frappe
-from frappe.utils import cstr, flt, nowdate, nowtime
+from frappe.utils import cstr, flt, now, nowdate, nowtime
 
 from erpnext.controllers.stock_controller import create_repost_item_valuation_entry
 
@@ -174,6 +174,7 @@
 			bin.set(field, flt(value))
 			mismatch = True
 
+	bin.modified = now()
 	if mismatch:
 		bin.set_projected_qty()
 		bin.db_update()