Update benches project to correct target framework

benches/Jellyfin.Common.Benches/Jellyfin.Common.Benches.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
   </PropertyGroup>
 
   <ItemGroup>