textproc/py-mkdocs-material: Update to 9.5.21

Changelogs since 9.5.18:

https://github.com/squidfunk/mkdocs-material/releases/tag/9.5.21
https://github.com/squidfunk/mkdocs-material/releases/tag/9.5.20
https://github.com/squidfunk/mkdocs-material/releases/tag/9.5.19

MFH: No (requires newer version textproc/py-mkdocs)