Document removal of browser reducer

Author: manisandro

src/release_notes/QWC2UpgradeNotes.md

@@ -24,6 +24,13 @@ Update to qwc2 submodule revision [574fe2a](https://github.com/qgis/qwc2/tree/57
 +      {"key": "Authentication", "mode": "Login", "icon": "login"}
 ```
 
+Update to qwc2 submodule revision [5705361](https://github.com/qgis/qwc2/tree/5705361) (11.12.2024) \[2024-lts → 2025-lts\]
+---------------------------------------------------------------------------------------------------
+
+*Note*: These changes are only relevant for custom plugin code. No configuration change is necessary.
+
+The `browser` actions/reducer have been removed. To determine whether the application is running on a mobile device, use `ConfigUtils.isMobile()`.
+
 Update to qwc2 submodule revision [52fe6ab](https://github.com/qgis/qwc2/tree/52fe6ab) (11.12.2024) \[2024-lts → 2025-lts\]
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