[GUI] Fix for the missing close icon and warnings under Unity 2023

[Eideren]

yeh

Unity removed that internal style in version 2023.3.0b10 at least

[nukeandbeans]

A cleaner solution to this would be to use (Texture2D)EditorGUIUtility.TrIconContent( "d_clear@2x" ).image to use the built-in icon. This will remove the need for extra icons in the package, and use the icons built into Unity. That icon is present all the way back to 2021.3, and probably before. Most likely won't go away because it's used in other styles (search clear boxes, for example).

I would also like to mention that I am currently in the process of rewriting all of the UI to use the new Overlays system, which will move the minimum version up to 2021.3, and most likely cause this PR to be short-lived. Progress on this can be tracked in the RCSG Discord. I'll leave this PR up for everyone else to see how to fix it for themselves, but for now I won't merge this (and other PRs, most likely). I do appreciate the effort, though. Thanks!