upgrade System.Memory dependency to 4.5.3

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -30,7 +30,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Memory" Version="4.5.2"/>
+    <PackageReference Include="System.Memory" Version="4.5.3"/>
     <PackageReference Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" Version="1.0.0"/>
     <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0"/>