ci: bump peaceiris/actions-mdbook from 1.1.14 to 1.2.0 (#793)

Bumps [peaceiris/actions-mdbook](https://github.com/peaceiris/actions-mdbook) from 1.1.14 to 1.2.0.
- [Release notes](https://github.com/peaceiris/actions-mdbook/releases)
- [Changelog](https://github.com/peaceiris/actions-mdbook/blob/main/CHANGELOG.md)
- [Commits](https://github.com/peaceiris/actions-mdbook/compare/v1.1.14...v1.2.0)

---
updated-dependencies:
- dependency-name: peaceiris/actions-mdbook
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

.github/workflows/test.yml

@@ -89,7 +89,7 @@ jobs:
 
       - name: Setup mdBook
         if: ${{ github.ref == 'refs/heads/main' }}
-        uses: peaceiris/actions-mdbook@v1.1.14
+        uses: peaceiris/actions-mdbook@v1.2.0
         with:
           mdbook-version: '0.4.5'