[Documentation] Changing suggested SKU to Standard_D8s_v5

[cptanalatriste]

✅ Checklist

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🚦 Depends on

⤴️ Summary

At the moment, the documentation suggests using a Standard_D8s_v6 for a single concurrent user. However, the Dsv6 sizes are in preview and not generally available.

In this PR, we change that suggestion to Standard_D8s_v5.

🌂 Related issues

Closes #2271

🔬 Tests

az vm list-sizes --location "uksouth" includes Standard_D8s_v5 at the time of writing.

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[craddm]

LGTM