Bump target frameworks used in unit tests

`netcoreapp3.1` is no longer supported or receiving security updates, so has been removed.

Author: drewnoakes

src/NetMQ.Tests/NetMQ.Tests.csproj

@@ -7,7 +7,7 @@
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
     <IsTestProject>true</IsTestProject>
-    <TargetFrameworks>net6.0;netcoreapp3.1;net47</TargetFrameworks>
+    <TargetFrameworks>net7.0;net48</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -26,7 +26,7 @@
     <AssemblySearchPaths Condition="'$(BaseFrameworkPathOverrideForMono)' != ''">$(FrameworkPathOverride)/Facades;$(AssemblySearchPaths)</AssemblySearchPaths>
   </PropertyGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net47' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net48' ">
     <Reference Include="mscorlib" />
     <Reference Include="System" />
     <Reference Include="System.Core" />
@@ -48,7 +48,7 @@
     </PackageReference>
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net47' ">
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net48' ">
     <PackageReference Include="ZeroMQ" Version="4.1.0.31" />
   </ItemGroup>