docs: update ECS service and Fargate documentation for new AZ rebalancing default values

[djglaser]

Updates documentation to reflect ECS change in default behavior for the AvailabilityZoneRebalancing property:

• For create service requests, when no value is specified for AvailabilityZoneRebalancing, Amazon ECS defaults to ENABLED if the ECS service is compatible with AvailabilityZoneRebalancing. If the ECS service is not compatible with AvailabilityZoneRebalancing, Amazon ECS defaults to DISABLED.
• For update service requests, when no value is specified for AvailabilityZoneRebalancing, Amazon ECS defaults to the existing service’s AvailabilityZoneRebalancing value. If the service never had an AvailabilityZoneRebalancing value set, Amazon ECS treats this as DISABLED.

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• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: d4113f8
• Result: SUCCEEDED
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