Upgrade guide note on GoogleMapsOverlays rightclick (#7753)

docs/upgrade-guide.md

@@ -6,6 +6,7 @@
 
 - `TextLayer`'s `maxWidth` is now relative to the text size. This change aims to make this prop more intuitive to use. If you have been using this prop in previous versions, divide its value by `64` (or `fontSettings.fontSize` if it's set manually).
 - `BitmapLayer` now handles translucent pixels correctly. In previous versions alpha was applied twice, leading to overly dim colors.
+- `GoogleMapsOverlays` now also triggers `onClick` for rightclick events. To filter out these events check for `event.srcEvent.domEvent.button === 2` in `onClick`.
 
 
 ## Upgrading from deck.gl v8.7 to v8.8