Rename old FIXME to TODO (#13646)

* Rename old FIXME to TODO, so we can track new FIXME starting v13

* Remove empty TODO

src/Umbraco.Core/IO/PhysicalFileSystem.cs

@@ -321,7 +321,7 @@ namespace Umbraco.Cms.Core.IO
             var originalPath = path;
             path = EnsureDirectorySeparatorChar(path);
 
-            // FIXME: this part should go!
+            // TODO: this part should go!
             // not sure what we are doing here - so if input starts with a (back) slash,
             // we assume it's not a FS relative path and we try to convert it... but it
             // really makes little sense?