Update mkdocs requirement from <1.4,>=1.1 to >=1.1,<1.5

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@1.1...1.4.0)

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

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

setup.py

@@ -52,7 +52,7 @@
     },
     install_requires=[
         "GitPython>=3.1,<3.2",
-        "mkdocs>=1.1,<1.4",
+        "mkdocs>=1.1,<1.5",
     ],
     # metadata
     keywords="mkdocs rss git plugin",