MINOR: [C#] Bump Grpc.Tools from 2.69.0 to 2.70.0 in /csharp (#45654)

Bumps [Grpc.Tools](https://github.com/grpc/grpc) from 2.69.0 to 2.70.0.
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Authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Signed-off-by: Curt Hagenlocher <[email protected]>

Author: dependabot[bot]

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

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Grpc.Tools" Version="2.69.0" PrivateAssets="All" />
+    <PackageReference Include="Grpc.Tools" Version="2.70.0" PrivateAssets="All" />
   </ItemGroup>
 
   <ItemGroup>

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

@@ -7,7 +7,7 @@
   <ItemGroup>
     <PackageReference Include="Google.Protobuf" Version="3.29.3" />
     <PackageReference Include="Grpc.Net.Client" Version="2.65.0" />
-    <PackageReference Include="Grpc.Tools" Version="2.69.0" PrivateAssets="All" />
+    <PackageReference Include="Grpc.Tools" Version="2.70.0" PrivateAssets="All" />
     <PackageReference Include="System.Memory" Version="4.6.0" />
   </ItemGroup>