Bump mkdocs-material from 9.1.7 to 9.1.8 (#357)

Author: daniel-zullo-frequenz

pyproject.toml

@@ -51,7 +51,7 @@ docs-gen = [
     "mike == 1.1.2",
     "mkdocs-gen-files == 0.4.0",
     "mkdocs-literate-nav == 0.6.0",
-    "mkdocs-material == 9.1.7",
+    "mkdocs-material == 9.1.8",
     "mkdocs-section-index == 0.3.5",
     "mkdocstrings[python] == 0.21.2",
 ]