chore(deps-dev): bump @contentful/app-sdk from 4.26.3 to 4.28.0 (#2039)

Bumps [@contentful/app-sdk](https://github.com/contentful/ui-extensions-sdk) from 4.26.3 to 4.28.0.
- [Release notes](https://github.com/contentful/ui-extensions-sdk/releases)
- [Changelog](https://github.com/contentful/ui-extensions-sdk/blob/main/CHANGELOG.md)
- [Commits](contentful/ui-extensions-sdk@v4.26.3...v4.28.0)

---
updated-dependencies:
- dependency-name: "@contentful/app-sdk"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]