Upgrade to .NET Core 3.1

[Alexey Knyazev]

https://dotnet.microsoft.com/platform/support/policy/dotnet-core

[Alexey Knyazev]

Besides simply changing TargetFramework to netcoreapp3.1, several other updates could be done here:

• It should be possible to switch to the new System.Text.Json APIs.
  • Migration guide
• It looks like floating number formatting has slightly changed since .NET Core 2.2:

  Floating point APIs have been updated to comply with IEEE 754-2008 revision. The goal of the .NET Core floating point project is to expose all “required” operations and ensure that they are behaviorally compliant with the IEEE spec.

  • Fewer decimal digits are now written for some values. Validation passes anyway.
  • Sign is now written for -0.0, see diff for Output\Positive\Node_Attribute\README.md

[Alexey Knyazev]

@bghgary What do you think about retargeting this project? .NET Core 2.2 reached its EoL about a year ago.

Given that glTF-Asset-Generator is an app rather than a library, we could aim as high as net5.0.

[Gary Hsu]

Sure, I don't see any issues with that.

[Alexey Knyazev]

Upgraded to .NET 5.0 in #564.