fix(accordion): update tabindex based on disabled state (#30986)

Issue number: resolves internal

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## What is the current behavior?
When an accordion item was set as disabled, it was still possible to
focus using keyboard navigation.

## What is the new behavior?
When an accordion item is disabled, `tabindex` is set as -1 to stop
being focusable using the keyboard navigation.

## Does this introduce a breaking change?

- [ ] Yes
- [ ] No

## Other information


[preview](https://ionic-framework-qznd9tqne-ionic1.vercel.app/src/components/accordion-group/test/states/)

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Co-authored-by: Maria Hutt <maria.hutt@outsystems.com>

Author: os-davidlourenco

core/src/components/accordion/accordion.tsx

@@ -514,6 +514,7 @@ export class Accordion implements ComponentInterface {
 
           'accordion-animated': this.shouldAnimate(),
         }}
+        tabindex={disabled ? '-1' : undefined}
       >
         <div
           onClick={() => this.toggleExpanded()}