Document --sdk-auth as deprecated

In #19949 we can read
> `--sdk-auth` was there since Nov 14, 2017 (https://github.com/Azure/azure-cli/commit/8346147d0da04063bd9000d1b370b2d65204cda8 from https://github.com/Azure/azure-cli/pull/5217). Azure SDK is not consuming it anymore, so we marked it as deprecated in https://github.com/Azure/azure-cli/pull/19414.

> Please simply forget about it and do not use it in any way.
>
> _Originally posted by @jiasli in https://github.com/Azure/azure-cli/issues/19949#issuecomment-948194997_

This has been/is used by many build systems, and it's still documented without any hint of being deprecated
on https://docs.microsoft.com/en-us/cli/azure/ad/sp?view=azure-cli-latest#az_ad_sp_create_for_rbac

So think this needs to be clearly documented there, and what should be used instead with i.e. GitHub Action for Azure CLI which has this documented as their preferred way
https://github.com/Azure/cli#configure-azure-credentials-as-github-secret

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* ID: e99d25ff-03e2-e015-37f4-a29b59ec50d4
* Version Independent ID: 1a8e88ed-72c7-fbe7-b0ad-68f23b3946cd
* Content: [az ad sp](https://docs.microsoft.com/en-us/cli/azure/ad/sp?view=azure-cli-latest#az_ad_sp_create_for_rbac)
* Content Source: [latest/docs-ref-autogen/ad/sp.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/main/latest/docs-ref-autogen/ad/sp.yml)
* Service: **active-directory**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**