Update .NET SDK version and bump NuGet package and Azure.Identity versions

[anamnavi]

PR Summary

This pull request updates dependencies to ensure the project uses the latest supported versions, which can provide important bug fixes, security patches, and new features.

Dependency updates:

• Updated all NuGet.* package references in Microsoft.PowerShell.PSResourceGet.csproj from version 6.9.1 to 6.14.3.
• Updated Azure.Identity package reference in Microsoft.PowerShell.PSResourceGet.csproj from version 1.14.2 to 1.21.0.
• Updated the .NET SDK version in global.json from 8.0.419 to 8.0.421.

PR Context

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[adityapatwardhan]

Check if bumping NuGet works, AFAIK there were issues with it and hence we kept on the 6.9.x version