chore(deps): update dependency mkdocs-material to v9.1.21

This MR contains the following updates:

Package	Update	Change
mkdocs-material	patch	==9.1.19 -> ==9.1.21

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Release Notes

squidfunk/mkdocs-material

v9.1.21: mkdocs-material-9.1.21

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• Fixed MkDocs 1.4 compat issue in social plugin (9.1.20 regression)

v9.1.20: mkdocs-material-9.1.20

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• Updated Sanskrit translations
• Fixed deprecation warnings for social plugin

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