C#: Replace usage of deprecated `project_settings_changed` signal

Replace usage of `EditorPlugin::project_settings_changed` signal with `ProjectSettings::settings_changed`.

modules/mono/editor/GodotTools/GodotTools/GodotSharpEditor.cs

@@ -440,7 +440,7 @@ namespace GodotTools
         {
             base._EnablePlugin();
 
-            ProjectSettingsChanged += GodotSharpDirs.DetermineProjectLocation;
+            ProjectSettings.SettingsChanged += GodotSharpDirs.DetermineProjectLocation;
 
             if (Instance != null)
                 throw new InvalidOperationException();