Update dependency mkdocs-material to v9.1.15

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
mkdocs-material	==9.1.14 -> ==9.1.15

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Release Notes

squidfunk/mkdocs-material

### [`v9.1.15`](https://togithub.com/squidfunk/mkdocs-material/releases/tag/9.1.15): mkdocs-material-9.1.15 [Compare Source](https://togithub.com/squidfunk/mkdocs-material/compare/9.1.14...9.1.15) - Fixed [#​5566](https://togithub.com/squidfunk/mkdocs-material/issues/5566): Indicate color scheme to operating system - Fixed [#​5565](https://togithub.com/squidfunk/mkdocs-material/issues/5565): Update `Dockerfile` to latest version of base image - Fixed [#​5554](https://togithub.com/squidfunk/mkdocs-material/issues/5554): Add additional version tags (`9`, `9.1`) to Docker image - Fixed [#​5536](https://togithub.com/squidfunk/mkdocs-material/issues/5536): Strip tags of ARIA labels in table of contents

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