Sort PRs with a single `platform:` label under "Porting:", even if not using `topic:porting` label

[akien-mga]

This is up for discussion, but when I was doing the changelogs manually, I would sort any platform-specific PR under Platforms/Porting, even if it doesn't explicitly have the topic:porting label.

Here's an example from 4.1-beta3 of a topic:rendering PR godotengine/godot#78235 which is Windows specific, and would IMO best be listed as "Porting: Windows: Fix for Win+M crashing the editor"

I don't know yet if it would always be the right decision though. For example it's up for debate when a PR like godotengine/godot#78120 should go under "Porting: Windows:" or "Buildsystem:". Intuitively I would say "Buildsystem:" is better, but maybe that's the only exception to the above rule?