docs: upgrade mkdocs to fix dark mode custom highlight style (#1695)

Author: Alexander Schueren

Diff for: docs/requirements.txt

@@ -1,4 +1,4 @@
 mike==1.1.2
-mkdocs-material==9.0.2
+mkdocs-material==9.3.2
 mkdocs-git-revision-date-plugin==0.3.2
 mkdocs-exclude==1.0.2

Diff for: docs/stylesheets/extra.css

@@ -3,10 +3,8 @@
 }
 
 .highlight .hll {
-    background-color: lavender;
-
-    [data-md-color-scheme="slate"] {
-        background-color: rgb(69, 48, 164)
+    [data-md-color-scheme="default"] {
+        background-color: lavender;
     }
 }