* Updated all NuGet packages to latest, and updated the framework from .NET Core 3.0 to 3.1.
* Updating appveyor settings for 3.1
Updating appveyor to use the netcoreapp3.1 path instead of 3.0.
* Removing unneeded NuGet package System.Runtime.CompilerServices.Unsafe.
* Removing unused NuGet package SharpFontCore.
* Removing unused NuGet package TimeZoneConverter.Posix
* Cleaning up by adding newline to a csproj.
* Simplfying a NuGet conditional include, and adding a warning disable for an annoying NuGet package.
* I'm not sure if .travis.yml is still used, but I'm updating its 'dotnet' version to the correct SDK.
* Making the runtime version into its own environment variable so it's a bit easier to change in the future.
* Removing OpenTK.NetStandard reference from Ryujinx.Common
* Fixing indentation in Common.csproj
* Updating the README to specify .NET Core 3.1.
* Reverting the update of the GTKSharp package so it doesn't block the PR.
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