UI: Update zIndex on NotificationList to fix the notification not being clickable in certain cases

[yoshi2no]

Closes #24476

What I did

Modified the z-index value in NotificationList from 10 to 11 to ensure the NotificationList always closes when the X button is pressed.

The reason I changed the z-index to 11 is because the z-index of the line section that allows for width adjustment of the side menu was set to 10. If there's a more appropriate value to be set, I'd like to change it to that.

before	after
https://github.com/storybookjs/storybook/assets/57059705/9ed4f961-353b-4b77-a051-efe550cdecc1	https://github.com/storybookjs/storybook/assets/57059705/12be530b-642c-4a80-8e15-7ee0ae132dbd

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

1. Run a sandbox for template, react-vite-default-ts
2. In code/lib/manager-api/src/modules/whatsnew.ts, do the following to intentionally display notifications.

  const initModule = async () => {

   // remove if statement
    fullAPI.addNotification({
      id: WHATS_NEW_NOTIFICATION_ID,
      link: '/settings/whats-new',
      content: {
        headline: 'issue #24476',
        subHeadline: "Click to learn what's new in Storybook",
      },
      icon: { name: 'hearthollow' },
      onClear({ dismissed }) {
        if (dismissed) {
          setWhatsNewCache({ lastDismissedPost: 'https://' });
        }
      },
    });
  };

3. Verify that the notification disappears when the X button is clicked.

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• Add or update documentation reflecting your changes
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  MIGRATION.MD

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  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
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  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
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[valentinpalkovic]

@cdedreuille Can you take this?

[cdedreuille]

LGTM. Thank you for making that change @yoshi2no