Add z-index to Notification Toast ListWrapper

[amelako]

Purpose

The Notification Toast didn't have a z-index value set, which caused stacking issues.

Approach and changes

• Added a new z-index value for the toast to design-tokens.
• Add z-index to ToastContext ListWrapper.

Definition of done

• Development completed
• Reviewers assigned
• Unit and integration tests
• Meets minimum browser support
• Meets accessibility requirements

[sumup-clark]

Hey @amelako,
we are super excited that you are contributing! 🎉There's one more thing you need to do. Please accept our Contributor License Agreement. It helps you and us to collaborate on clear terms and focus on what we love most: code.

Thanks!

[changeset-bot]

🦋 Changeset detected

Latest commit: 1ef3075

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

This PR includes changesets to release 2 packages

Name	Type
@sumup/circuit-ui	Major
@sumup/design-tokens	Minor

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Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

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🔍 Inspect: https://vercel.com/sumup/oss-circuit-ui/6W6aAik6X5WBRAeh3uWct8UNnn33
✅ Preview: https://oss-circuit-ui-git-fix-toast-stacking-sumup.vercel.app

[codecov]

Codecov Report

Merging #1359 (1ef3075) into main (ab62a9e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1359   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         190      190           
  Lines        3782     3782           
  Branches     1209     1209           
=======================================
  Hits         3487     3487           
  Misses        276      276           
  Partials       19       19           

Impacted Files	Coverage Δ
...ircuit-ui/components/ToastContext/ToastContext.tsx	92.30% <ø> (ø)
packages/design-tokens/themes/shared.ts	0.00% <ø> (ø)
packages/design-tokens/utils/theme-prop-type.ts	0.00% <ø> (ø)