Bump @antora/pdf-extension in /playbook

Bumps [@antora/pdf-extension](https://gitlab.com/antora/antora-assembler) from 1.0.0-alpha.7 to 1.0.0-alpha.9.
- [Changelog](https://gitlab.com/antora/antora-assembler/blob/main/CHANGELOG.adoc)
- [Commits](https://gitlab.com/antora/antora-assembler/compare/v1.0.0-alpha.7...v1.0.0-alpha.9)

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

Signed-off-by: dependabot[bot] <support@github.com>

playbook/package.json

@@ -13,7 +13,7 @@
     "@antora/cli": "^3.1.7",
     "@antora/collector-extension": "^1.0.0-alpha.3",
     "@antora/lunr-extension": "^1.0.0-alpha.8",
-    "@antora/pdf-extension": "^1.0.0-alpha.7",
+    "@antora/pdf-extension": "^1.0.0-alpha.9",
     "@antora/site-generator": "^3.1.7",
     "asciidoctor-highlight.js": "^0.4.0",
     "asciidoctor-kroki": "^0.18.1",