Deprecate iotcentral command group

[r-delgadillo]

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Thank you for contributing to the IoT extension!

This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• If introducing new functionality or modified behavior, are they backed by unit and integration tests?
• In the same context as above are command names and their parameter definitions accurate? Do help docs have sufficient content?
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[r-delgadillo]

Command group 'az iotcentral' has been deprecated on the Azure CLI. Deprecating the same command group in extension.

PR for Azure CLI
https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/HISTORY.rst

[ghost]

should we remove this iotcentral app too

[r-delgadillo]

We have to leave the command there for at least 3 months before removing it.

[r-delgadillo]

The changes in this PR will mark anything under az iotcentral as deprecated

[digimaun]

If possible we should. Also the deprecated iotcentral device-twin command group. @r-delgadillo is there a reason they are still around?

[r-delgadillo]

We have a work item in our backlog to completely remove all commands under az iotcentral in June or July, I forgot which month specifically. We'd like to keep these commands available for customers who are using them to give them sufficient time for them to roll off these commands and start using the new command group. Hence that's the reason we are hiding the existing command group so no other customers become aware of those commands.