test(textarea): use legacy form syntax for legacy test (#27196)

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## What is the current behavior?
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The legacy test template for the textarea test does not use a label and
does not specify `legacy="true"`.

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Issue URL: N/A


## What is the new behavior?
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this PR. -->

- Adds the `legacy="true"` property to the legacy template

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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core/src/components/textarea/test/legacy/basic/index.html

@@ -35,7 +35,7 @@
           </ion-item>
 
           <ion-item>
-            <ion-textarea placeholder="Textarea with no label"></ion-textarea>
+            <ion-textarea placeholder="Textarea with no label" legacy="true"></ion-textarea>
           </ion-item>
 
           <ion-item>