feat: support `multi_az_with_standby_enabled` for opensearch

[lukehsiao]

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.

Closes: https://github.com/cloudposse/terraform-aws-elasticsearch/issues/195

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what

This PR simply exposes a new variable (multi_az_with_standby_enabled) for OpenSearch clusters.

why

This is the recommended setting by AWS, so it makes sense to be able to do this via terraform.

references

Closes: #195