Ensure latest engine version is used for AWS MQ Brokers

Not using latest mq version increases the likelihood of getting exploited.

In AWS Console -

1. Sign in to the AWS Console and go to the MQ dashboard.
2. In the navigation panel, Select Brokers.
3. In the Broker details tab select Edit.
4. Under Maintenance, Enable automatic minor version upgrades.

In Terraform -

1. In the aws_mq_broker resource, set the engine_version field to the latest version number.

References:
https://docs.aws.amazon.com/amazon-mq/latest/developer-guide/upgrading-brokers.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/mq_broker#engine_version