Update supported platforms note for "use system accent color" option

[Joseph-DiGiovanni]

Thanks to #104106 I believe all platforms that have accent colors now have the feature implemented, making this note unnecessary.

This PR is for the tool-tip of the theme option in the editor, not the display server documentation.

[Mickeon]

I think the only thing that could be removed here should be "only" in the note, because we tend to use "only" only when a single platform supports the subject in question.

[AThousandShips]

"Only implemented" is used for multiple cases 6 times, 8 times for a single OS, so there's no such strong tendency

For "only supported" it's 7 for multiple, and 3 for single cases

There's other combinations I didn't look at, but there's no issue with "only" here IMO

[Joseph-DiGiovanni]

I'm personally in favor of removing "only" when the list has grown as long as it has. We may be better off waiting for godotengine/godot-proposals#11993, as it should bring web theme detection to parity with other platforms that the editor currently supports. Then, I believe this note can be safely removed.

[Calinou]

In the long term, we'll probably want to add a platforms="windows,linux" attribute to documentable items in the class reference, similar to what #63079 does for notes and warnings (although that one is more free-form). This way, we can standardize these notes and reduce the amount of work needed for translators.