Added release note for radio group and button widget improvements

Author: PPakalns

release-content/release-notes/radio_group_and_button_widget_improvements.md

@@ -0,0 +1,20 @@
+---
+title: "`RadioButton`, `RadioGroup` widget minor improvements"
+authors: ["@PPakalns"]
+pull_requests: [21294]
+---
+
+`RadioButton` and `RadioGroup` usage remains fully backward compatible.
+
+Improvements:
+- Event propagation from user interactions will now be canceled even if
+  widgets are disabled. Previously, some relevant event propagation 
+  was not properly canceled.
+- `RadioButton` now emits a `ValueChange<bool>` entity event when checked, 
+  even when checked via a `RadioGroup`. Consistent with other `Checkable` widgets.
+  As a `RadioButton` cannot be unchecked through direct user interaction with this widget, 
+  a `ValueChange` event with value `false` can not be triggered for `RadioButton`.
+- If a `RadioButton` is focusable, a value change event can be triggered
+  using the **Space** or **Enter** keys when focused.
+- `RadioGroup` is now optional and can be replaced with a custom implementation.
+