References updates

Updates for Rx, ReactiveUI and Microsoft CSharp nuget references.

build/Microsoft.CSharp.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="Microsoft.CSharp" Version="4.3.0" />
+    <PackageReference Include="Microsoft.CSharp" Version="4.5.0" />
   </ItemGroup>
 </Project>

build/Microsoft.Reactive.Testing.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="Microsoft.Reactive.Testing" Version="3.0.0" />
+    <PackageReference Include="Microsoft.Reactive.Testing" Version="4.1.0" />
   </ItemGroup>
 </Project>

build/ReactiveUI.props

@@ -1,5 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="reactiveui" Version="8.0.0" />
+    <PackageReference Include="reactiveui" Version="8.7.1" />
   </ItemGroup>
 </Project>

build/Rx.props

@@ -1,9 +1,9 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="3.1.1" />
-    <PackageReference Include="System.Reactive.Core" Version="3.1.1" />
-    <PackageReference Include="System.Reactive.Interfaces" Version="3.1.1" />
-    <PackageReference Include="System.Reactive.Linq" Version="3.1.1" />
-    <PackageReference Include="System.Reactive.PlatformServices" Version="3.1.1" />
+    <PackageReference Include="System.Reactive" Version="4.1.0" />
+    <PackageReference Include="System.Reactive.Core" Version="4.1.0" />
+    <PackageReference Include="System.Reactive.Interfaces" Version="4.1.0" />
+    <PackageReference Include="System.Reactive.Linq" Version="4.1.0" />
+    <PackageReference Include="System.Reactive.PlatformServices" Version="4.1.0" />
   </ItemGroup>
 </Project>