DSD-1742: Fixing z-index issue on MultiSelect selected items count button

[EdwinGuzman]

Fixes JIRA ticket DSD-1742

This PR does the following:

• Fixes the selected item count button's z-index issue.

How has this been tested?

Storybook, specifically check the layout patterns Story in the MultiSelectGroup for a better example where various MultiSelect components overlap.

Accessibility concerns or updates

• N/A

Checklist:

• [ ] I have updated the Storybook documentation accordingly.
• [ ] I have added relevant accessibility documentation for this pull request.
• [ ] All new and existing tests passed.

Front End Review:

• [ ] Review the Vercel preview deployment once it is ready.

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Name	Status	Preview	Comments	Updated (UTC)
nypl-design-system	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 16, 2024 2:32pm