Use dotnet-public feed instead of NuGet.org for NuGet packages in Dockerfiles

[lbussell]

Fixes #1286

AI generated summary

This pull request updates how NuGet packages are sourced in Dockerfiles and sample projects, switching downloads from the public NuGet.org feed to the Azure DevOps dotnet-public feed. This improves reliability and consistency for package retrieval, especially for Roslyn compilers and reference assemblies. It also adds explicit NuGet configuration to sample apps and removes a permissive network policy from pipeline configuration.

NuGet package source updates:

• Changed all Roslyn compiler package downloads in Dockerfiles (microsoft.net.compilers.2.9.0.nupkg, microsoft.net.compilers.3.6.0.nupkg) to use the Azure DevOps dotnet-public feed instead of NuGet.org, across all relevant templates and image definitions.
• Updated reference assembly package installation in SDK Dockerfiles to use the dotnet-public feed for Microsoft.NETFramework.ReferenceAssemblies.* NuGet packages.
• Added new variables for package URLs and sources in manifest.versions.json to centralize feed configuration.

Sample project improvements:

• Added NuGet.config files to sample projects (aspnetapp, aspnetmvcapp, dotnetapp) to explicitly point to the dotnet-public feed, and updated Dockerfiles to copy these configs for reliable restores.

Pipeline configuration:

• Removed the permissive network isolation policy from pipeline YAML files, as builds now use the Azure DevOps feed and no longer require broad network access.