Updated target frameworks

[tomvanenckevort]

Types of Changes

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• [x] I have read the CONTRIBUTING document
• [x] My code follows the code style of this project

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• [ ] Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• [x] New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [x] All new and existing tests passed

Description

Updated the target frameworks for the project to be in line with AngleSharp (as discussed in #89).

[FlorianRappl]

The build is currently failing:

The current .NET SDK does not support targeting .NET 7.0. Either target .NET 6.0 or lower, or use a version of the .NET SDK that supports .NET 7.0. [/home/runner/work/AngleSharp.Js/AngleSharp.Js/src/AngleSharp.Js.Tests/AngleSharp.Js.Tests.csproj]

I think what needs to change is the installation of the frameworks. I've made the change in the devel branch.