Drop the correct deleted branch table (#26028)

There's a typo in #22743
The correct table name is `deleted_branch` not `deleted_branches`

models/migrations/v1_21/v264.go

@@ -89,5 +89,5 @@ func AddBranchTable(x *xorm.Engine) error {
 		}
 	}
 
-	return x.DropTables("deleted_branches")
+	return x.DropTables(new(DeletedBranch))
 }

models/migrations/v1_21/v269.go

@@ -0,0 +1,12 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_21 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func DropDeletedBranchTable(x *xorm.Engine) error {
+	return x.DropTables("deleted_branch")
+}