Use low-privilege secrets variable group for DockerHub access

[lbussell]

Fixes https://github.com/dotnet/dotnet-docker-internal/issues/8828.

Important note: "unofficial" != "low privilege". That's why the template is still called secrets-unofficial instead of secrets-low.

• Low privilege secrets are safe to reference from any pipeline, they shouldn't give any write access anywhere.
• Unofficial secrets may give write access to non-production resources.

The reference to the DotNet-Docker-Secrets-Unofficial variable group was only removed because the unofficial pipelines don't currently use it for anything. We might add it back in the future to enable more functionality in unofficial pipelines.