Update mkdocs requirement from ~=1.5 to ~=1.6

Updates the requirements on [mkdocs](https://github.com/mkdocs/mkdocs) to permit the latest version.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.5.0...1.6.0)

---
updated-dependencies:
- dependency-name: mkdocs
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements_docs.txt

@@ -1,5 +1,5 @@
 mike~=2.0
-mkdocs~=1.5
+mkdocs~=1.6
 mkdocs-awesome-pages-plugin~=2.9
 mkdocs-material~=9.5; python_version>="3.8"
 mkdocs-material~=9.2; python_version<"3.8"