Update mobile example categories for Qt 6.6

Add mobile example category tags for the chosen examples.
Add for the ones that work best for both, Android and iOS.
Remove from the ones that do not work for both (at least yet).

Added
- hellogles3
Removed
- qtandroidnotifier (not iOS)

Fixes: QTBUG-116780
Pick-to: 6.6
Change-Id: Icb4c7ba1f22d4e67089a565071d83cdf8ec95896
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>

examples/corelib/platform/androidnotifier/doc/src/androidnotifier-example.qdoc

@@ -4,7 +4,6 @@
 /*!
     \title Qt Android Notifier
     \example platform/androidnotifier
-    \examplecategory {Mobile}
     \meta tag {widgets,android,notification}
     \brief Demonstrates calling Java code from Qt in an Android application.
 

examples/opengl/doc/src/hellogles3.qdoc

@@ -6,6 +6,7 @@
     \title Hello GLES3 Example
     \ingroup examples-widgets-opengl
     \examplecategory {Graphics & Multimedia}
+    \examplecategory {Mobile}
 
     \brief Demonstrates OpenGL ES 3.0 functions via QOpenGLExtraFunctions.