Remove the deprecated shadow style mixins

[robinmetral]

Purpose

Remove the deprecated shadowSingle, shadowDouble and shadowTriple style mixins.

Approach and changes

• Remove the deprecated shadowSingle, shadowDouble and shadowTriple style mixins
• Migrate a remaining use of shadowDouble in the TabList component to the shadow style mixin.
• ⚠️ Note: this PR doesn't include a codemod. The change should be verified visually, and we expect the current use of deprecated shadow mixins to be flow.

Definition of done

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

[sumup-clark]

Hey @robinmetral,
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: 5303124

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

This PR includes changesets to release 1 package

Name	Type
@sumup/circuit-ui	Major

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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/7T9oJ3DqafqSerbQmaZwfTDiz8i7
✅ Preview: https://oss-circuit-ui-git-remove-legacy-shadow-mixins.sumup-vercel.app

[codecov]

Codecov Report

Merging #1510 (5303124) into canary (c7dfe6a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           canary    #1510   +/-   ##
=======================================
  Coverage   92.57%   92.58%           
=======================================
  Files         201      201           
  Lines        4216     4207    -9     
  Branches     1353     1353           
=======================================
- Hits         3903     3895    -8     
+ Misses        293      292    -1     
  Partials       20       20           

Impacted Files	Coverage Δ
...t-ui/components/Tabs/components/TabList/TabList.js	100.00% <ø> (ø)
packages/circuit-ui/styles/style-mixins.ts	98.94% <ø> (-0.10%)	⬇️
...mponents/NotificationBanner/NotificationBanner.tsx	83.60% <0.00%> (+1.63%)	⬆️

[robinmetral]

☝️ resolved the merge conflicts in the migration guide via the GH UI. Wish it supported a clean rebase, but 🤷