Add some reference to .net5

[NeilMacMullen]

It would be useful to update this page to include a reference to .net5 (and, in future .net6) since as it stands it looks like only .net core 3.1 is supported from the current text. Adding ".net5" would also help the page appear in search results - I spent several hours searching for combinations of "net5", "vcxproj", "C++", "TargetFrameworkVersion" etc but never hit this.

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Document Details

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• ID: 590dc11f-354e-ded2-469f-605febf6b394
• Version Independent ID: 097bfdc7-9d3d-852f-eba3-e7ea2e0c4239
• Content: Migrating C++/CLI projects to .NET Core - .NET Core
• Content Source: docs/core/porting/cpp-cli.md
• Product: dotnet-fundamentals
• GitHub Login: @mjrousos
• Microsoft Alias: dotnetcontent

[adegeo]

I believe this was fixed in #28850