feat: support multi_az_with_standby_enabled for opensearch

Note that this bumps the minimum `hashicorp/aws` provider version to
5.15.0, where this parameter was introduced [[1]].

The README diff was generated with `make init` and `make readme`, and
introduces some minor unrelated changes.

[1]: https://github.com/hashicorp/terraform-provider-aws/blob/main/CHANGELOG.md#5150-august-31-2023
Closes: #195

Author: lukehsiao

README.md

@@ -129,13 +129,13 @@ Available targets:
 | Name | Version |
 |------|---------|
 | <a name="requirement_terraform"></a> [terraform](#requirement\_terraform) | >= 1.3 |
-| <a name="requirement_aws"></a> [aws](#requirement\_aws) | >= 3.35.0 |
+| <a name="requirement_aws"></a> [aws](#requirement\_aws) | >= 5.15.0 |
 
 ## Providers
 
 | Name | Version |
 |------|---------|
-| <a name="provider_aws"></a> [aws](#provider\_aws) | >= 3.35.0 |
+| <a name="provider_aws"></a> [aws](#provider\_aws) | >= 5.15.0 |
 
 ## Modules
 
@@ -238,6 +238,7 @@ Available targets:
 | <a name="input_log_publishing_index_enabled"></a> [log\_publishing\_index\_enabled](#input\_log\_publishing\_index\_enabled) | Specifies whether log publishing option for INDEX\_SLOW\_LOGS is enabled or not | `bool` | `false` | no |
 | <a name="input_log_publishing_search_cloudwatch_log_group_arn"></a> [log\_publishing\_search\_cloudwatch\_log\_group\_arn](#input\_log\_publishing\_search\_cloudwatch\_log\_group\_arn) | ARN of the CloudWatch log group to which log for SEARCH\_SLOW\_LOGS needs to be published | `string` | `""` | no |
 | <a name="input_log_publishing_search_enabled"></a> [log\_publishing\_search\_enabled](#input\_log\_publishing\_search\_enabled) | Specifies whether log publishing option for SEARCH\_SLOW\_LOGS is enabled or not | `bool` | `false` | no |
+| <a name="input_multi_az_with_standby_enabled"></a> [multi\_az\_with\_standby\_enabled](#input\_multi\_az\_with\_standby\_enabled) | Enable domain with standby for OpenSearch cluster | `bool` | `false` | no |
 | <a name="input_name"></a> [name](#input\_name) | ID element. Usually the component or solution name, e.g. 'app' or 'jenkins'.<br/>This is the only ID element not also included as a `tag`.<br/>The "name" tag is set to the full `id` string. There is no tag with the value of the `name` input. | `string` | `null` | no |
 | <a name="input_namespace"></a> [namespace](#input\_namespace) | ID element. Usually an abbreviation of your organization name, e.g. 'eg' or 'cp', to help ensure generated IDs are globally unique | `string` | `null` | no |
 | <a name="input_node_to_node_encryption_enabled"></a> [node\_to\_node\_encryption\_enabled](#input\_node\_to\_node\_encryption\_enabled) | Whether to enable node-to-node encryption | `bool` | `false` | no |