update(rss): use 1.17.9 as RSS minimal version to use upstream fix

Author: Guts

requirements.txt

@@ -9,4 +9,4 @@ pre-commit>=4.5.1,<5
 mkdocs-glightbox>=0.5.2,<0.6
 mkdocs-material[imaging]>=9.7.1,<9.8
 mkdocs-redirects>=1.2.2,<1.3
-mkdocs-rss-plugin>=1.17.8,<1.18
+mkdocs-rss-plugin>=1.17.9,<1.18