bug: ion-button should be disabled when used inside disabled fieldset

# Bug Report

**Ionic version:**
[x] **5.6.8**

**Current behavior:**
`` inside a disabled `` remains not disabled.

**Expected behavior:**
`` inside a disabled `` should become disabled. This is the behavior of a native `` when placed inside a disabled ``.

**Steps to reproduce:**
Put an `` inside a ``. ([CodePen](https://codepen.io/SepehrSepahi/pen/zYZeVJQ))

**Related code:**
```html

Disabled Fieldset
Native Button
Ion-Button

```

Start with the ion-button component and reproduce the issue using the linked CodePen or a fieldset disabled example. Compare its behavior with the native button, then verify that ion-button becomes disabled when placed inside a disabled fieldset.

由索引模型根据 Issue 内容生成。