docs: update breaking changes for v21 (#35820)

docs/breaking-changes.md

@@ -80,7 +80,7 @@ win.webContents.on('input-event', (_, event) => {
 })
 ```
 
-## Planned Breaking API Changes (20.0)
+## Planned Breaking API Changes (21.0)
 
 ### Behavior Changed: V8 Memory Cage enabled
 
@@ -144,6 +144,8 @@ webContents.printToPDF({
 })
 ```
 
+## Planned Breaking API Changes (20.0)
+
 ### Default Changed: renderers without `nodeIntegration: true` are sandboxed by default
 
 Previously, renderers that specified a preload script defaulted to being