ci(deps): update github actions "setup-dotnet" and "checkout" to v5 and use matrix syntax for dotnet version

[DevTKSS]

The second Step (Publish) could also use dotnet.matrix but I did not implement it until now, because it could be possible that building to net8.0 has a specific reason, like keeping the backwards compatibility

[Copilot]

Pull Request Overview

This PR modernizes the CI/CD pipeline by updating GitHub Actions to their latest major versions (v5) and refactoring the build job to use a matrix strategy for testing against multiple .NET versions (8.x and 9.x). This ensures the project is tested against both supported .NET versions while reducing workflow duplication.

• Updated actions/checkout and actions/setup-dotnet from v3 to v5
• Introduced matrix strategy to test builds against both .NET 8.x and 9.x in parallel
• Consolidated two separate .NET setup steps into a single matrix-driven step

Comments suppressed due to low confidence (2)

.github/workflows/build.yml:43

• The artifact name should include the matrix variable to avoid conflicts between jobs. With the matrix strategy, both .NET 8.x and 9.x jobs will try to upload artifacts with the same name 'nuget-packages', causing the second job to overwrite the first. Consider using 'nuget-packages-${{ matrix.dotnet }}' or have only one matrix configuration upload artifacts (e.g., using an 'if' condition).

                  name: nuget-packages

.github/workflows/build.yml:60

• The 'publish' job downloads artifacts with name 'nuget-packages', but the build job now runs as a matrix and may create multiple artifact names. The artifact name in the download step needs to be updated to match the upload artifact naming strategy, or the build job should upload artifacts only once (e.g., from a single matrix configuration).

                  name: nuget-packages

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