chore(build): Remove net6.0 references

[rhysparry]

This pull request updates the .NET Core version used in the CI workflows and modifies the target frameworks for the test project. The most important changes include:

CI Workflow Updates:

• .github/workflows/dotnet-ci-build.yml: Removed setup for .NET Core 6.
• .github/workflows/nuget-publish.yml: Removed setup for .NET Core 6.

Project Configuration:

• Dirt.Args.Tests/Dirt.Args.Tests.csproj: Changed the target frameworks from net8.0;net6.0 to net8.0 for the test project.

Resolves: #38

[rhysparry]

This change is part of the following stack:

• chore(build): Remove net6.0 references #43 ◀

_{Change managed by git-spice.}