Update dependency mkdocs-material to v9.6.21

| datasource | package         | from   | to     |
| ---------- | --------------- | ------ | ------ |
| pypi       | mkdocs-material | 9.6.20 | 9.6.21 |


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

Author: renovate[bot]

docs/requirements.txt

@@ -1,7 +1,7 @@
 mkdocs==1.6.1
 mike==2.1.3
 markdown==3.9
-mkdocs-material==9.6.20
+mkdocs-material==9.6.21
 
 # Markdown extensions
 Pygments==2.19.2