Downgrade System.Memory to 4.5.4 (#2579)

Author: ViktorHofer

Directory.Packages.props

@@ -24,7 +24,7 @@
     <PackageVersion Include="Microsoft.CSharp" Version="4.7.0" />
     <PackageVersion Include="Microsoft.DotNet.PlatformAbstractions" Version="3.1.6" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
-    <PackageVersion Include="System.Memory" Version="4.6.3" />
+    <PackageVersion Include="System.Memory" Version="4.5.4" />
     <PackageVersion Include="system.reactive.core" Version="6.0.0" />
   </ItemGroup>
 
@@ -34,4 +34,4 @@
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.4.3" />
   </ItemGroup>
 
-</Project>
+</Project>