Fix Microsoft.NETCore.App vulnerability warnings

[c72578]

Microsoft.NETCore.App 2.0.0 contains a vulnerability.

• Switch to version 2.0.9 by specifying RuntimeFrameworkVersion.
• Fixes the following warnings: Warning NU1903 Package 'Microsoft.NETCore.App' 2.0.0 has a known high severity vulnerability, https://github.com/advisories/GHSA-7mfr-774f-w5r9
• This only concerns the TargetFramework netcoreapp2.0 of: CUETools.ALACEnc/CUETools.ALACEnc.csproj CUETools.ARCUE/CUETools.ARCUE.csproj CUETools.CTDB.Converter/CUETools.CTDB.Converter.csproj CUETools.Converter/CUETools.Converter.csproj CUETools.Flake/CUETools.Flake.csproj CUETools.LossyWAV/CUETools.LossyWAV.csproj CUETools.eac3to/CUETools.eac3to.csproj