* infra: Migrate to .NET 5
This migrate projects and CI to .NET 5
* Remove language version restrictions (now on 9.0 by default)
* infra: pin .NET 5 to avoid later issues
* infra: Cleanup csproj files
* infra: update dependencies
* infra: Add temporary workaround for a bug in Vector128.Create
see https://github.com/dotnet/runtime/issues/44704 for more informations
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