repository CI/CD pipelines use ubuntu-16.04 images, which will be removed on 18 October 2021

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)

Repository build pipeilne

### Description of issue (in as much detail as possible)

Per the warnings that show on build validation pipelines right now (see [my PR check pipeline](https://dev.azure.com/azure-sdk/public/_build/results?buildId=1127632&view=results), the warnings tab), [azure-pipelines.yml](https://github.com/Azure/azure-cli-extensions/blob/main/azure-pipelines.yml) is using ubuntu-16.04 images for checkout. These images will be removed in less than 14 days, at which point the PR validation pipeline will break.

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