build(deps): bump prosemirror-model from 1.19.4 to 1.21.1

Bumps [prosemirror-model](https://github.com/prosemirror/prosemirror-model) from 1.19.4 to 1.21.1.
- [Changelog](https://github.com/ProseMirror/prosemirror-model/blob/master/CHANGELOG.md)
- [Commits](ProseMirror/prosemirror-model@1.19.4...1.21.1)

---
updated-dependencies:
- dependency-name: prosemirror-model
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>