Analyzer template: netcoreapp2.0 is out of support

After this step https://docs.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix#create-the-solution, I get a compilation warning because test project uses `netcoreapp2.0` which is out of support.

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#### Document Details

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* ID: 6e3fc905-7aa1-7f5e-9bfe-fbcef461ad5e
* Version Independent ID: a286e20b-1e43-dc57-e0fb-5a9dee7ec3f6
* Content: [Tutorial: Write your first analyzer and code fix](https://docs.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix)
* Content Source: [docs/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix.md](https://github.com/dotnet/docs/blob/main/docs/csharp/roslyn-sdk/tutorials/how-to-write-csharp-analyzer-code-fix.md)
* Product: **dotnet-csharp**
* Technology: **csharp-roslyn**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**