fix(backdrop): remove tabindex for improved accessibility (#29956)

Issue number: resolves #29773

---------

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## What is the current behavior?
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Certain Chrome and Edge versions (confirmed: Chrome v127 and Edge v127)
would indicate that the backdrop has an accessibility violation:

```
Blocked aria-hidden on a <ion-backdrop> element because the element that just received
focus must not be hidden from assistive technology users. Avoid using aria-hidden on a
focused element or its ancestor. Consider using the inert attribute instead, which will also
prevent focus. For more details, see the aria-hidden section of the WAI-ARIA specification at
```

The error is happening because `tabindex` and `aria-hidden` are being
passed to `ion-backdrop`. The `tabindex` attribute is used to make an
element focusable, regardless of value. When `aria-hidden` is applied to
an element, then the element is hidden from screen readers. This
violates the accessibility standards since `ion-backdrop` would be
considered a focusable element but not visible to screen readers.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Removed `tabindex`, this aligns with frameworks known for
accessibility (Chakra UI)

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: `8.3.4-dev.11729533091.1ac77a0c`

How to test:

1. Use either Chrome v127 or Edge v127
2. Navigate to the [alert basic
page](https://ionic-framework-3pvgcj4b9-ionic1.vercel.app/src/components/alert/test/basic)
from the `main` branch
3. Open browser console
4. Click on the first button on the alert page
5. Click on the backdrop
6. Notice the error message in the browser console (if you don't see it,
then use a private browser or clear cache)
7. Navigate to the [alert basic
page](https://ionic-framework-git-rou-11175-ionic1.vercel.app/src/components/alert/test/basic)
from the `ROU-11175` branch
8. Repeat steps 2-6
9. Verify that the error does not appear

core/src/components/backdrop/backdrop.tsx

@@ -51,7 +51,6 @@ export class Backdrop implements ComponentInterface {
     const mode = getIonMode(this);
     return (
       <Host
-        tabindex="-1"
         aria-hidden="true"
         class={{
           [mode]: true,