refactor(textarea): remove legacy property and support for legacy syntax (#28993)

Issue number: internal

---------

## What is the current behavior?

In Ionic Framework v7, we [simplified the textarea
syntax](https://ionic.io/blog/ionic-7-is-here#simplified-form-control-syntax)
so that it was no longer required to be placed inside of an `ion-item`.
We maintained backwards compatibility by adding a `legacy` property
which allowed it to continue to be styled properly when written in the
following way:

```html
<ion-item>
  <ion-label>Label</ion-label>
  <ion-textarea></ion-textarea>
</ion-item>
```

While this was supported in v7, console warnings were logged to notify
developers that they needed to update this syntax for the best
accessibility experience.

## What is the new behavior?

- Removes the `legacy` property and support for the legacy syntax.
Developers should follow the [migration
guide](https://ionicframework.com/docs/api/textarea#migrating-from-legacy-textarea-syntax)
in the textarea documentation to update their apps. The new syntax
requires a `label` or `aria-label` on `ion-textarea`:
    ```html
    <ion-item>
      <ion-textarea label="Label"></ion-textarea>
    </ion-item>
    ```
- Removes the legacy tests under `textarea/test/legacy/` and all related
screenshots
- Removes the textarea usage in `input/test/legacy/spec`,
`item/test/disabled`, `item/test/legacy/disabled` and
`item/test/legacy/fill`

## Does this introduce a breaking change?

- [x] Yes
- [ ] No

1. Developers have had console warnings when using the legacy syntax
since the v7 release. The migration guide for the new textarea syntax is
outlined in the [Textarea
documentation](https://ionicframework.com/docs/api/textarea#migrating-from-legacy-textarea-syntax).
2. This change has been documented in the Breaking Changes document with
a link to the migration guide.

BREAKING CHANGE:

The `legacy` property and support for the legacy syntax, which involved
placing an `ion-textarea` inside of an `ion-item` with an `ion-label`,
have been removed from textarea. For more information on migrating from
the legacy textarea syntax, refer to the [Textarea
documentation](https://ionicframework.com/docs/api/textarea#migrating-from-legacy-textarea-syntax).

---------

Co-authored-by: ionitron <hi@ionicframework.com>

core/src/components/input/test/legacy/spec/index.html

@@ -160,26 +160,6 @@
           <ion-label position="stacked">Align items: center</ion-label>
           <div class="ion-align-self-end">A div: align self right</div>
         </ion-item>
-        <ion-item>
-          <ion-label position="floating">Floating: textarea</ion-label>
-          <ion-textarea></ion-textarea>
-        </ion-item>
-        <ion-item>
-          <ion-label position="floating">Floating: textarea value</ion-label>
-          <ion-textarea value="value"></ion-textarea>
-        </ion-item>
-        <ion-item>
-          <ion-label position="stacked">Stacked: textarea</ion-label>
-          <ion-textarea></ion-textarea>
-        </ion-item>
-        <ion-item class="item-has-focus">
-          <ion-label position="stacked">Stacked: textarea focused value</ion-label>
-          <ion-textarea value="value"></ion-textarea>
-        </ion-item>
-        <ion-item class="custom item-has-focus">
-          <ion-label position="stacked">Stacked: textarea focus</ion-label>
-          <ion-textarea></ion-textarea>
-        </ion-item>
       </ion-content>
 
       <style>