chore(deps): bump @tiptap/extension-placeholder (#5382)

Bumps [@tiptap/extension-placeholder](https://github.com/ueberdosis/tiptap/tree/HEAD/packages/extension-placeholder) from 2.8.0 to 2.9.1.
- [Release notes](https://github.com/ueberdosis/tiptap/releases)
- [Changelog](https://github.com/ueberdosis/tiptap/blob/develop/packages/extension-placeholder/CHANGELOG.md)
- [Commits](https://github.com/ueberdosis/tiptap/commits/@tiptap/[email protected]/packages/extension-placeholder)

---
updated-dependencies:
- dependency-name: "@tiptap/extension-placeholder"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>