Update dependency mkdocs-material to v9.1.18 (#714)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [mkdocs-material](https://togithub.com/squidfunk/mkdocs-material) |
patch | `==9.1.17` -> `==9.1.18` |

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### Release Notes

<details>
<summary>squidfunk/mkdocs-material (mkdocs-material)</summary>

###
[`v9.1.18`](https://togithub.com/squidfunk/mkdocs-material/releases/tag/9.1.18):
mkdocs-material-9.1.18

[Compare
Source](https://togithub.com/squidfunk/mkdocs-material/compare/9.1.17...9.1.18)

-   Updated Danish translations
-   Added support for installing user requirements in Docker image
- Fixed
#&#8203;[squidfunk/mkdocs-material#5655):
Search separator with lookbehind breaks highlighting

</details>

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.github/workflows/mkdocs-requirements.txt

@@ -7,7 +7,7 @@ Markdown==3.4.3
 MarkupSafe==2.1.3
 mkdocs==1.4.3
 mkdocs-macros-plugin==1.0.1
-mkdocs-material==9.1.17
+mkdocs-material==9.1.18
 mkdocs-material-extensions==1.1.1
 Pygments==2.15.1
 pymdown-extensions==10.0.1