Bump antora from 3.2.0-alpha.4 to 3.2.0-alpha.6 (#2501)

Bumps [antora](https://gitlab.com/antora/antora) from 3.2.0-alpha.4 to 3.2.0-alpha.6.
- [Changelog](https://gitlab.com/antora/antora/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora/compare/v3.2.0-alpha.4...v3.2.0-alpha.6)

---
updated-dependencies:
- dependency-name: antora
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

package.json

@@ -1,6 +1,6 @@
 {
   "dependencies": {
-    "antora": "3.2.0-alpha.4",
+    "antora": "3.2.0-alpha.6",
     "@antora/atlas-extension": "1.0.0-alpha.2",
     "@antora/collector-extension": "1.0.0-alpha.3",
     "@asciidoctor/tabs": "1.0.0-beta.6",