fix(flutter): Add missing experimental for privacy options before GA (#14014)

Author: philprime

platform-includes/replay/privacy-configuration/dart.flutter.mdx

@@ -17,9 +17,9 @@ For example, you can explicitly mask or unmask widgets by type,
 or you can even have a callback to decide whether a specific widget instance should be masked:
 
 ```dart
-  options.privacy.mask<IconButton>();
-  options.privacy.unmask<Image>();
-  options.privacy.maskCallback<Text>(
+  options.experimental.privacy.mask<IconButton>();
+  options.experimental.privacy.unmask<Image>();
+  options.experimental.privacy.maskCallback<Text>(
       (Element element, Text widget) =>
           (widget.data?.contains('secret') ?? false)
               ? SentryMaskingDecision.mask