chore(release): v1.9.0RC1

TODO:
- [ ] Do `tx pull -a --minimum-perc=100` as a separate `feature(i18n): ...` commit before the release commit.
- [ ] Rerun the release script from scratch (should fix composer.json).
- [ ] Manually merge 1.9.0-rc.1 section with 1.9.0-dev section since there was no -dev release.