fix(checkbox): set aria-checked of indeterminate checkbox to 'mixed' (#29115)

Issue number: resolves Internal

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->
We are not ever explicitly setting `aria-checked`. For checked and
unchecked states (i.e. `true` and `false` for aria-checked), we don't
need to set `aria-checked` because an input with a type of 'checkbox'
has built-in semantics making `aria-checked` redundant.

However, when the checkbox is in an indeterminate state, `aria-checked`
should have a value of 'mixed'. We are not currently ever setting it to
'mixed'. See
[MDN](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-checked#description)
for more details.

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

- The checkbox's `aria-checked` has a value of 'true' when it is checked
- The checkbox's `aria-checked` has a value of 'false' when it is
unchecked
- The checkbox's `aria-checked` has a value of 'mixed' when it is
indeterminate

## 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. -->

core/src/components/checkbox/checkbox.tsx

@@ -251,6 +251,7 @@ export class Checkbox implements ComponentInterface {
 
     return (
       <Host
+        aria-checked={indeterminate ? 'mixed' : `${checked}`}
         class={createColorClasses(color, {
           [mode]: true,
           'in-item': hostContext('ion-item', el),

core/src/components/checkbox/test/checkbox.spec.ts

@@ -39,3 +39,18 @@ describe('ion-checkbox: disabled', () => {
     expect(checkbox.checked).toBe(false);
   });
 });
+
+describe('ion-checkbox: indeterminate', () => {
+  it('should have a mixed value for aria-checked', async () => {
+    const page = await newSpecPage({
+      components: [Checkbox],
+      html: `
+        <ion-checkbox indeterminate="true">Checkbox</ion-checkbox>
+      `,
+    });
+
+    const checkbox = page.body.querySelector('ion-checkbox')!;
+
+    expect(checkbox.getAttribute('aria-checked')).toBe('mixed');
+  });
+});