Update max_staleness in google_bigquery_table to be a computed field (GoogleCloudPlatform#13167)

Author: wj-chen

mmv1/third_party/terraform/services/bigquery/resource_bigquery_table.go.tmpl

@@ -965,6 +965,7 @@ func ResourceBigQueryTable() *schema.Resource {
 			"max_staleness": {
 				Type:        schema.TypeString,
 				Optional:    true,
+				Computed:    true,
 				Description: `The maximum staleness of data that could be returned when the table (or stale MV) is queried. Staleness encoded as a string encoding of [SQL IntervalValue type](https://cloud.google.com/bigquery/docs/reference/standard-sql/data-types#interval_type).`,
 			},