Update .NET documentation to remove v3.1 and v5 reference, add v7.

[aureq]

What happened?

Our .NET documentation states "... you will need to install both Pulumi and .NET Core SDK 3.1, .NET 5, or .NET 6 (recommended)."

However, only .NET6 and 7 are actively maintained and supported by Microsoft. And with .NET 8 out in a few weeks.

We should update our documentation so users install the right version.

Example

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Output of pulumi about

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Additional context

Source ref: https://github.com/pulumi/pulumi-hugo/edit/master/themes/default/content/docs/languages-sdks/dotnet/_index.md

Contributing

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[Frassle]

I recently fixed a similar issue for our references to python versions (#3249), I suggest we take the same approach here. Nothing in our docs should list a specific version number because that requires us to keep coming back to try and keep them up to date. Instead just refer to Microsofts page for what's supported.

[aureq]

@Frassle If you could look at this please #3573