Update dependency mkdocs-material to v9.1.21

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
mkdocs-material	==9.1.20 -> ==9.1.21

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Release Notes

squidfunk/mkdocs-material (mkdocs-material)

### [`v9.1.21`](https://togithub.com/squidfunk/mkdocs-material/releases/tag/9.1.21): mkdocs-material-9.1.21 [Compare Source](https://togithub.com/squidfunk/mkdocs-material/compare/9.1.20...9.1.21) - Fixed MkDocs 1.4 compat issue in social plugin (9.1.20 regression)

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[codecov[bot]]

Codecov Report

Merging #789 (094d8c3) into master (38f6f61) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #789   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           14        14           
  Lines          834       834           
  Branches        62        62           
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  Hits           834       834           

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