Bump @antora/site-generator from 3.1.7 to 3.1.8 in /antora (#867)

Bumps [@antora/site-generator](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"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

antora/package.json

@@ -12,7 +12,7 @@
   "devDependencies": {
     "@antora/cli": "^3.1.7",
     "@antora/lunr-extension": "^1.0.0-alpha.8",
-    "@antora/site-generator": "^3.1.7",
+    "@antora/site-generator": "^3.1.8",
     "@antora/collector-extension": "^1.0.0-alpha.4",
     "asciidoctor-highlight.js": "^0.4.0",
     "asciidoctor-kroki": "^0.18.1",