refactor(core): remove global hidden attribute (#25829)

resolves #17583

BREAKING CHANGE: 

The `[hidden]` attribute has been removed from Ionic's global stylesheet. The `[hidden]` attribute can continue to be used, but developers will get the [native `hidden` implementation](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden) instead. The main difference is that the native implementation is easier to override using `display` than Ionic's implementation.

Developers can add the following CSS to their global stylesheet if they need the old behavior:

```css
[hidden] {
  display: none !important;
}
```

core/src/css/core.scss

@@ -195,8 +195,7 @@ ion-modal-controller,
 ion-picker-controller,
 ion-popover-controller,
 ion-toast-controller,
-.ion-page-hidden,
-[hidden] {
+.ion-page-hidden {
   /* stylelint-disable-next-line declaration-no-important */
   display: none !important;
 }