Upgrade hedgedoc to 1.9.0
There's been a couple of fairly impactful security upgrades, we should perform them.
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.8.0
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.8.1
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.8.2
- https://github.com/hedgedoc/hedgedoc/releases/tag/1.9.0
Migrated from T3574 (view on Phabricator)