fix(csharp): update project reference for nested submodule structure

Fix the path to Apache.Arrow.Adbc.Drivers.Apache.csproj to account for
the nested submodule structure in hiveserver2's main branch.

The path changed from:
  ..\arrow-adbc\csharp\src\Drivers\Apache\

To:
  ..\arrow-adbc\csharp\arrow-adbc\csharp\src\Drivers\Apache\

This is because hiveserver2's main branch has arrow-adbc as a nested
submodule at csharp/arrow-adbc/ rather than having the source directly
in the csharp/ directory.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: eric-wang-1990

csharp/src/AdbcDrivers.Databricks.csproj

@@ -19,6 +19,6 @@
     </Content>
   </ItemGroup>
   <ItemGroup>
-    <ProjectReference Include="..\arrow-adbc\csharp\src\Drivers\Apache\Apache.Arrow.Adbc.Drivers.Apache.csproj" />
+    <ProjectReference Include="..\arrow-adbc\csharp\arrow-adbc\csharp\src\Drivers\Apache\Apache.Arrow.Adbc.Drivers.Apache.csproj" />
   </ItemGroup>
 </Project>