chore: Update .NET SDK to version 8 (LTS)

[HofmeisterAn]

What does this PR do?

Updates the .NET SDK to version 8 (latest LTS).

Why is it important?

We are trying to run our tests utilizing the latest .NET SDK (LTS). Testcontainers itself, including the modules, still targets netstandard2.0 and netstandard2.1 to provide support for most .NET Framework and .NET versions.

Related issues

-

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[0xced]

Note that SourceLink is now included in the .NET SDK:

Source Link is now included in the .NET SDK. The goal is that by bundling Source Link into the SDK, instead of requiring a separate <PackageReference> for the package, more packages will include this information by default. That information will improve the IDE experience for developers.

Since the global.json file requires the .NET 8 SDK, all the package references to Microsoft.SourceLink.GitHub could be removed instead of updated to version 8.0.0.

[HofmeisterAn]

Thanks for sharing. I will remove it in the next few days 🧹.