textproc/py-mkdocs-material: Update to 9.6.18

Changelogs since 9.6.16:

https://github.com/squidfunk/mkdocs-material/releases/tag/9.6.18
https://github.com/squidfunk/mkdocs-material/releases/tag/9.6.17

MFH: 2025Q3
(cherry picked from commit bea09d3c95b8afa4113accb9105fba77b64c6250)