Upgrade Jellyfin.Model.Tests to net5.0

tests/Jellyfin.Model.Tests/Jellyfin.Model.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <Nullable>enable</Nullable>