Build(deps-dev): bump tinymce from 6.7.0 to 6.8.1

Bumps [tinymce](https://github.com/tinymce/tinymce/tree/HEAD/modules/tinymce) from 6.7.0 to 6.8.1.
- [Changelog](https://github.com/tinymce/tinymce/blob/develop/modules/tinymce/CHANGELOG.md)
- [Commits](https://github.com/tinymce/tinymce/commits/6.8.1/modules/tinymce)

---
updated-dependencies:
- dependency-name: tinymce
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>