Fix selected checkbox fill color when checkbox group is disabled

[yihuiliao]

Closes #5378

After investigating this a bit more, I don't think this is a css specificity issue. It seemed that when the isDisabled prop was provided to<CheckboxGroup>, that value was not passed to the individual <Checkbox> components, resulting in the absence of the is-disabled class in their classnames. Therefore, if you wanted to the disabled styles to be applied to a selected checkbox in a checkbox group, you would also have to directly pass isDisabled to that specific checkbox.

Looking back at the commit history, it appears that this "broke" with this PR (#3827). However, if you go commit before it (which is #3962), you'll notice that the styling might be correct, but the is-disabled class still isn't a part of the classname even though it should be.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

1. Go to the CheckboxGroup in storybook
2. Use the controls and play around the different states when isDisabled is true
3. It should have the correct fill color when it is disabled

🧢 Your Project:

[rspbot]

Build successful! 🎉

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[ktabors]

Tested on chrome and iphone and wasn't able to find a case where it wasn't correct.

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[rspbot]

## API Changes

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