textproc/py-mkdocs-material: Update to 9.6.21

• Notably with this version, the temporary pin for py-click, that was required because version 8.2.2 of it was yanked due regressions, has been lifted. [1]

• Remove no longer required patch.

Changelog since 9.6.18:

https://github.com/squidfunk/mkdocs-material/releases/tag/9.6.21
https://github.com/squidfunk/mkdocs-material/releases/tag/9.6.20
https://github.com/squidfunk/mkdocs-material/releases/tag/9.6.19

PR: 289603 [1]
Reported by: Thibault Payet [1]