Bump protobuf-net from 2.4.6 to 3.0.21

Bumps [protobuf-net](https://github.com/protobuf-net/protobuf-net) from 2.4.6 to 3.0.21.
- [Release notes](https://github.com/protobuf-net/protobuf-net/releases)
- [Changelog](https://github.com/protobuf-net/protobuf-net/blob/main/docs/releasenotes.md)
- [Commits](https://github.com/protobuf-net/protobuf-net/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

DepotDownloader/DepotDownloader.csproj

@@ -5,7 +5,7 @@
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="protobuf-net" Version="2.4.6" />
+    <PackageReference Include="protobuf-net" Version="3.0.21" />
     <PackageReference Include="SteamKit2" Version="2.3.0-beta.2" />
   </ItemGroup>
 </Project>