Update mkdocs-rss-plugin requirement from <1.8,>=1.4 to >=1.4,<1.9

Updates the requirements on [mkdocs-rss-plugin](https://github.com/Guts/mkdocs-rss-plugin) to permit the latest version.
- [Release notes](https://github.com/Guts/mkdocs-rss-plugin/releases)
- [Changelog](https://github.com/Guts/mkdocs-rss-plugin/blob/main/CHANGELOG.md)
- [Commits](Guts/mkdocs-rss-plugin@1.4.0...1.8.0)

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

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

requirements.txt

@@ -21,7 +21,7 @@ mkdocs-glightbox>=0.3,<0.4
 mkdocs-macros-plugin>=0.5,<1.1
 mkdocs-minify-plugin<0.7
 mkdocs-redirects<1.3
-mkdocs-rss-plugin>=1.4,<1.8
+mkdocs-rss-plugin>=1.4,<1.9
 
 # social plugin requirements
 CairoSVG>=2.5,<3