Upgrade assistant does not handle .slnx files

[paulloz]

Godot version

4.6.dev (0759141 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