Update from CoreOS to Flatcar linux for az vm images list

[jamesc]

As CoreOS is deprecated and there is no Image in the marketplace any more, update to use Flatcar linux instead in the list of available images.

There is already a PR (Azure/azure-rest-api-specs#14796) in the API Specs repo which holds the master list. We need to update our local fallback copy (https://github.com/Azure/azure-cli/blob/dev/src/azure-cli/azure/cli/command_modules/vm/_alias.py) so that this work in environments where endpoint_vm_image_alias_doc is not set.

[jamesc]

Once this is merged we should also update the example in https://docs.microsoft.com/en-us/azure/virtual-machines/linux/cli-ps-findimage

[yonzhan]

Add @dbradish-microsoft to help with updating the example in example in https://docs.microsoft.com/en-us/azure/virtual-machines/linux/cli-ps-findimage once this is merged.