Extend blacklisting of tst_QWidget::showMinimizedKeepsFocus to macOS 15

It's flaky on all version.

Pick-to: 6.8 6.9
Change-Id: I01b2f0877efc100578c55d69e9f9ed65c02b2aab
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

tests/auto/widgets/kernel/qwidget/BLACKLIST

@@ -14,8 +14,7 @@ sles-15
 # QTBUG-87668
 [showMinimizedKeepsFocus]
 android
-macos-13 ci
-macos-14 ci
+macos ci
 [normalGeometry]
 android
 [saveRestoreGeometry]