chore(deps): update dependency mkdocs-material to v9.6.21 (#736)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

requirements.txt

@@ -1 +1 @@
-mkdocs-material==9.6.20
+mkdocs-material==9.6.21