Bump @antora/site-generator-default from 3.1.7 to 3.1.8

Bumps [@antora/site-generator-default](https://gitlab.com/antora/antora) from 3.1.7 to 3.1.8.
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v3.1.7...v3.1.8)

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

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