Feature request: return old value for `az pipelines variable-group variable update`?

Thanks~

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#### Document Details

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* ID: 5b3c226e-dc10-db12-601c-e1502f5768ab
* Version Independent ID: 111beb11-a3ab-b288-7fcf-52b3944ad449
* Content: [az pipelines variable-group variable](https://docs.microsoft.com/en-us/cli/azure/ext/azure-devops/pipelines/variable-group/variable?view=azure-cli-latest#ext-azure-devops-az-pipelines-variable-group-variable-update)
* Content Source: [latest/docs-ref-autogen/ext/azure-devops/pipelines/variable-group/variable.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/ext/azure-devops/pipelines/variable-group/variable.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**

Start with the `az pipelines variable-group variable update` command and the extension implementation behind the documented `variable.yml` entry. Reproduce the current command behavior, then clarify the expected output and define done as returning the previous variable value without changing the update itself.

Written by the indexing model from the issue text.