Clarify .NET SDK requirements for test project target frameworks

[Copilot]

Description

Addresses review feedback on PR #27 regarding whether .NET 5.0.x, 6.0.x, and 7.0.x SDKs are necessary for running tests.

Finding: These SDKs are required. The test project (Wolfgang.Etl.Abstractions.Tests.Unit.csproj) explicitly targets net5.0, net6.0, and net7.0 alongside other frameworks. Each target framework requires its corresponding SDK to build and execute tests.

Removing these SDK versions from the CI workflow would require first removing the corresponding target frameworks from the test project's TargetFrameworks property.

Type of change

• Documentation update

How Has This Been Tested?

• Verified test project target frameworks in .csproj file
• Confirmed SDK requirements match workflow configuration

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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

Screenshots (if applicable)

N/A

Additional context

No code changes were made. This PR documents the analysis confirming the current CI workflow configuration is correct for the test project's multi-targeting requirements.

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