fix(modal, SWC-344): update background-color for s2 modal

[Rajdeepc]

Description

This PR fixes SWC-344. Updates the --spectrum-modal-background-color-quiet for modal

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

• [x] The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

• [x] VRTs have been run and looked at.
• [x] Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

Screenshots

Updated:

To-do list

• [ ] I have read the contribution guidelines.
• [ ] I have updated relevant storybook stories and templates.
• [ ] I have tested these changes in Windows High Contrast mode.
• [ ] If my change impacts other components, I have tested to make sure they don't break.
• [ ] If my change impacts documentation, I have updated the documentation accordingly.
• [ ] ✨ This pull request is ready to merge. ✨

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 768c10b2fd247818eb5c62b3fe5014071b0990a7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------------------- | ----- | | @spectrum-css/modal | Major |

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