CSS: ck-z-modal -> ck-z-panel (for v41.0.0)

See also:
- https://github.com/ckeditor/ckeditor5/blob/v41.0.0/CHANGELOG.md

  > ui: The --ck-z-modal CSS custom property was renamed to --ck-z-panel.
  > We recommend updating custom CSS and integrations that use this custom
  > property to avoid presentation issues. See #14973.
- https://github.com/ckeditor/ckeditor5/issues/14973

theme/mathform.css

@@ -31,5 +31,5 @@
 }
 
 .ck.ck-toolbar-container {
-	z-index: calc(var(--ck-z-modal) + 2);
+	z-index: calc(var(--ck-z-panel) + 2);
 }