deps: update mkdocs requirement from <1.6,>=1.2.3 to >=1.2.3,<1.7

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@1.2.3...1.6.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -1,4 +1,4 @@
-mkdocs >= 1.2.3, < 1.6
+mkdocs >= 1.2.3, < 1.7
 mkdocs-material >= 8.2.3, < 9.6
 mkdocstrings >= 0.18.0, < 1
 mkdocstrings[python] >= 0.18, < 1