docs: add missing vibrancy breaking change (#40905)

docs: add missing vibranch change

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <[email protected]>

docs/breaking-changes.md

@@ -45,6 +45,18 @@ systemPreferences.on('high-contrast-color-scheme-changed', () => { /* ... */ })
 nativeTheme.on('updated', () => { /* ... */ })
 ```
 
+### Removed: Some `window.setVibrancy` options on macOS
+
+The following vibrancy options have been removed:
+
+* 'light'
+* 'medium-light'
+* 'dark'
+* 'ultra-dark'
+* 'appearance-based'
+
+These were previously deprecated and have been removed by Apple in 10.15.
+
 ### Removed: `webContents.getPrinters`
 
 The `webContents.getPrinters` method has been removed. Use