MINOR: [C#] Bump Grpc.Net.Client and System.Runtime.CompilerServices.…

…Unsafe

Bumps [Grpc.Net.Client](https://github.com/grpc/grpc-dotnet) and [System.Runtime.CompilerServices.Unsafe](https://github.com/dotnet/runtime). These dependencies needed to be updated together.

Updates `Grpc.Net.Client` from 2.63.0 to 2.64.0
- [Release notes](https://github.com/grpc/grpc-dotnet/releases)
- [Changelog](https://github.com/grpc/grpc-dotnet/blob/master/doc/release_process.md)
- [Commits](grpc/grpc-dotnet@v2.63.0...v2.64.0)

Updates `System.Runtime.CompilerServices.Unsafe` from 4.7.1 to 6.0.0
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](https://github.com/dotnet/runtime/commits/v6.0.0)

---
updated-dependencies:
- dependency-name: Grpc.Net.Client
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: System.Runtime.CompilerServices.Unsafe
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

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

@@ -6,7 +6,7 @@
 
   <ItemGroup>
     <PackageReference Include="Google.Protobuf" Version="3.27.2" />
-    <PackageReference Include="Grpc.Net.Client" Version="2.63.0" />
+    <PackageReference Include="Grpc.Net.Client" Version="2.64.0" />
     <PackageReference Include="Grpc.Tools" Version="2.65.0" PrivateAssets="All" />
   </ItemGroup>