MINOR: [C#] Bump Google.Protobuf from 3.28.2 to 3.28.3 in /csharp

Bumps [Google.Protobuf](https://github.com/protocolbuffers/protobuf) from 3.28.2 to 3.28.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/protobuf_release.bzl)
- [Commits](protocolbuffers/protobuf@v3.28.2...v3.28.3)

---
updated-dependencies:
- dependency-name: Google.Protobuf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

csharp/src/Apache.Arrow.Flight/Apache.Arrow.Flight.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.28.2" />
+    <PackageReference Include="Google.Protobuf" Version="3.28.3" />
     <PackageReference Include="Grpc.Net.Client" Version="2.65.0" />
     <PackageReference Include="Grpc.Tools" Version="2.67.0" PrivateAssets="All" />
     <PackageReference Include="System.Memory" Version="4.5.5" />

csharp/test/Apache.Arrow.Flight.TestWeb/Apache.Arrow.Flight.TestWeb.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Google.Protobuf" Version="3.28.2" />
+    <PackageReference Include="Google.Protobuf" Version="3.28.3" />
     <PackageReference Include="Grpc.AspNetCore" Version="2.66.0" />
   </ItemGroup>