chore: Only include the System.Memory dependency in netstandard builds

Signed-off-by: Jon Skeet <[email protected]>

Author: jskeet

src/CloudNative.CloudEvents/CloudNative.CloudEvents.csproj

@@ -9,7 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Memory" Version="4.5.5" />
+    <PackageReference Include="System.Memory" Version="4.5.5" Condition="'$(TargetFramework)'=='netstandard2.0' or '$(TargetFramework)'=='netstandard2.1'" />
     <!-- Source-only package with nullable reference annotations. -->
     <PackageReference Include="Nullable" Version="1.3.1" PrivateAssets="All" />
   </ItemGroup>