.NET: Remove SourceLink package reference

[WeihanLi]

Motivation and Context

Starting with .NET 8, Source Link for the following source control providers is included in the .NET SDK and enabled by default:

GitHub or GitHub Enterprise
Azure Repos git repositories (formerly known as Visual Studio Team Services)
GitLab 12.0+ (for older versions see GitLab settings)
Bitbucket 4.7+ (for older versions see Bitbucket settings)
If your project uses .NET SDK 8+ and is hosted by the above providers it does not need to reference any Source Link packages or set any build properties to include source control information in the PDB.

https://github.com/dotnet/sourcelink

Description

Removed SourceLink package reference since not necessary

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR removes the Microsoft.SourceLink.GitHub package reference from the NuGet package configuration, as it's no longer needed for .NET 8+ projects hosted on GitHub (SourceLink is now included in the SDK by default).

Key Changes:

• Removed the Microsoft.SourceLink.GitHub package reference and associated comments from nuget-package.props