Migrate Project to .NET 8: Update Target Framework.

[gmottajr]

Migration to .NET 8: .csproj files are being updated.

• \C-Sharp\Algorithms\Algorithms.csproj
• \C-Sharp\Algorithms.Tests\Algorithms.Tests.csproj
• \C-Sharp\DataStructures\DataStructures.csproj
• \C-Sharp\DataStructures.Tests\DataStructures.Tests.csproj
• \C-Sharp\Utilities\Utilities.csproj
• \C-Sharp\Utilities.Tests\Utilities.Tests.csproj

Please let me know whether or not it would be necessary to create new unit tests for this change.

Motivation and Context

Github issue #425: Proposed to migrate to .NET 8 to use the latest features of .NET ecosystem. It was requested to perform the changes in a few PRs to make them smaller and easier to review. This is the one only meant to update .NET version.

• [X] I have performed a self-review of my code
• [X] My code follows the style guidelines of this project
• [X] All existing unit tests have been executed, and all have passed successfully.
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Comments in areas I changed are up to date
• [ ] I have added comments to hard-to-understand areas of my code
• [X] I have made corresponding changes to the README.md