Bump protobuf-net from 3.4.14 to 3.4.21 (#731)

---
updated-dependencies:
- dependency-name: protobuf-net
  dependency-version: 3.4.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

DepotDownloader/DepotDownloader.csproj

@@ -25,7 +25,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="protobuf-net" Version="3.4.14" />
+    <PackageReference Include="protobuf-net" Version="3.4.21" />
     <PackageReference Include="QRCoder" Version="1.8.0" />
     <PackageReference Include="SteamKit2" Version="3.4.0" />
   </ItemGroup>