Doc: Replace Qt 5 QtGraphicalEffects code snippet with Qt 6 MultiEffect

Qt Graphical Effects was deprecated in Qt 6 and replaced with the
MultiEffect type. Fix code snippets in Qt 6 documentation to use the
MultiEffect QML type instead of the deprecated Qt Graphical Effects
type.

Fixes: QTBUG-119992
Pick-to: 6.5
Change-Id: I27c11ed013880978656f86b6322bf3e4bfa18812
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
(cherry picked from commit a95df6a234)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 134261baf9)

src/quickcontrols/doc/src/qtquickcontrols-fileselectors.qdoc

@@ -77,7 +77,7 @@
     \code
     // +Material/CustomButton.qml
     import QtQuick
-    import QtGraphicalEffects
+    import QtQuick.Effects
     import QtQuick.Controls
     import QtQuick.Controls.Material
 
@@ -93,11 +93,12 @@
             radius: width / 2
 
             layer.enabled: control.enabled
-            layer.effect: DropShadow {
-                verticalOffset: 1
-                color: Material.dropShadowColor
-                samples: control.pressed ? 20 : 10
-                spread: 0.5
+            layer.effect: MultiEffect {
+                shadowEnabled: true
+                shadowHorizontalOffset: 3
+                shadowVerticalOffset: 3
+                shadowColor: Material.dropShadowColor
+                shadowBlur: control.pressed ? 0.8 : 0.4
             }
         }
     }