Revert back to netcoreapp2.1

Jellyfin.Server/Jellyfin.Server.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <AssemblyName>jellyfin</AssemblyName>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>netcoreapp2.2;netcoreapp3.0</TargetFrameworks>
+    <TargetFramework>netcoreapp2.2</TargetFramework>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>