Upgrade assistant does not handle .slnx files

### Godot version

4.6.dev ([`0759141`](https://github.com/raulsntos/godot/commit/075914121d) from new-dotnet-module-plus-source-code-plugin-wip)

### Godot .NET packages version

4.6.dev (7b5efa1 from upgrade-assistant-plus-source-code-plugin-wip)

### System information

Windows 10

### .NET information

SDK 9.0.306

### Issue description

Running the upgrade assistant on a project defining its solution with a `.slnx` file instead of a `.sln` one results in an error. The path is correct, but we hard-code the file extension in `get_project_sln_path()`.
```
File does not exist: '/correct/path/to/solution/file/X.sln'.

ERROR: .NET failed to upgrade project 'X'. See output above for more details.
at: Upgrade (\godot-dotnet\src\Godot.EditorIntegration\UpgradeAssistant\GodotUpgradeAssistant.cs:442)
\godot-dotnet\src\Godot.EditorIntegration\UpgradeAssistant\GodotUpgradeAssistant.cs:442 - .NET failed to upgrade project 'X'. See output above for more details.
```

### Steps to reproduce

- Create a blank project.
- Migrate the `.sln` to a `.slnx`.
- Open the editor.

### Minimal reproduction project

N/A

Start in GodotUpgradeAssistant.cs around line 442 and locate get_project_sln_path(), which the issue identifies as hard-coding the solution extension. Reproduce with a project migrated from .sln to .slnx, then verify the upgrade assistant accepts the .slnx path without the file-not-found error.

Written by the indexing model from the issue text.