Bump @antora/site-generator from 3.1.8 to 3.1.9

Bumps [@antora/site-generator](https://gitlab.com/antora/antora) from 3.1.8 to 3.1.9.
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v3.1.8...v3.1.9)

---
updated-dependencies:
- dependency-name: "@antora/site-generator"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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