fix(assuredworkloads): update the API

#### assuredworkloads:v1

The following keys were changed:
- schemas.GoogleCloudAssuredworkloadsV1Workload.properties.billingAccount.description

discovery/assuredworkloads-v1.json

@@ -351,7 +351,7 @@
       }
     }
   },
-  "revision": "20210906",
+  "revision": "20210916",
   "rootUrl": "https://assuredworkloads.googleapis.com/",
   "schemas": {
     "GoogleCloudAssuredworkloadsV1CreateWorkloadOperationMetadata": {
@@ -425,7 +425,7 @@
       "id": "GoogleCloudAssuredworkloadsV1Workload",
       "properties": {
         "billingAccount": {
-          "description": "Required. Input only. The billing account used for the resources which are direct children of workload. This billing account is initially associated with the resources created as part of Workload creation. After the initial creation of these resources, the customer can change the assigned billing account. The resource name has the form `billingAccounts/{billing_account_id}`. For example, `billingAccounts/012345-567890-ABCDEF`.",
+          "description": "Optional. The billing account used for the resources which are direct children of workload. This billing account is initially associated with the resources created as part of Workload creation. After the initial creation of these resources, the customer can change the assigned billing account. The resource name has the form `billingAccounts/{billing_account_id}`. For example, `billingAccounts/012345-567890-ABCDEF`.",
           "type": "string"
         },
         "complianceRegime": {

src/apis/assuredworkloads/v1.ts

@@ -333,7 +333,7 @@ export namespace assuredworkloads_v1 {
    */
   export interface Schema$GoogleCloudAssuredworkloadsV1Workload {
     /**
-     * Required. Input only. The billing account used for the resources which are direct children of workload. This billing account is initially associated with the resources created as part of Workload creation. After the initial creation of these resources, the customer can change the assigned billing account. The resource name has the form `billingAccounts/{billing_account_id\}`. For example, `billingAccounts/012345-567890-ABCDEF`.
+     * Optional. The billing account used for the resources which are direct children of workload. This billing account is initially associated with the resources created as part of Workload creation. After the initial creation of these resources, the customer can change the assigned billing account. The resource name has the form `billingAccounts/{billing_account_id\}`. For example, `billingAccounts/012345-567890-ABCDEF`.
      */
     billingAccount?: string | null;
     /**