Update dependency protobuf-net to 3.4.21

Author: renovate[bot]

src/Lagrange.XocMat.Adapter/Lagrange.XocMat.Adapter.csproj

@@ -7,7 +7,7 @@
     </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="protobuf-net" Version="3.2.56" />
+    <PackageReference Include="protobuf-net" Version="3.4.21" />
     <PackageReference Include="SixLabors.ImageSharp" Version="3.1.12" />
   </ItemGroup>
 </Project>