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 backend 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 backend treats this as DISABLED.

Issue # (if applicable)

Reintroduces #35156 (was reverted by #35233 due to regression)

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Description of changes

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Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[vishaalmehrishi]

The pr/do-not-merge label will be removed once we get the go-ahead from the service team.

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[kumvprat]

@Mergifyio requeue

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Pull request has been modified.

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