Bump mhausenblas/mkdocs-deploy-gh-pages from 1.20 to 1.21

Bumps [mhausenblas/mkdocs-deploy-gh-pages](https://github.com/mhausenblas/mkdocs-deploy-gh-pages) from 1.20 to 1.21.
- [Release notes](https://github.com/mhausenblas/mkdocs-deploy-gh-pages/releases)
- [Commits](https://github.com/mhausenblas/mkdocs-deploy-gh-pages/compare/1.20...1.21)

```
updated-dependencies:
- dependency-name: mhausenblas/mkdocs-deploy-gh-pages
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Change-Id: I8f5be517028ffa5c6798e4a6adc223fc8204ed98
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 938e462..fe3a0da 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -15,6 +15,6 @@
           fetch-depth: 0
 
       - name: Deploy docs
-        uses: mhausenblas/mkdocs-deploy-gh-pages@1.20
+        uses: mhausenblas/mkdocs-deploy-gh-pages@1.21
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}