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

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
mkdocs-material	==9.1.17 -> ==9.1.21

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Release Notes

squidfunk/mkdocs-material (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

v9.1.19: mkdocs-material-9.1.19

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• Added support for MkDocs 1.5+
• Fixed #​5699: Improve error reporting in social plugin

v9.1.18: mkdocs-material-9.1.18

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• Updated Danish translations
• Added support for installing user requirements in Docker image
• Fixed #​5655: Search separator with lookbehind breaks highlighting

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