Remove MkDocs 2 warning

This is from Material for MkDocs, warning that it will not support
MkDocs 2.

Material for MkDocs has pinned to `mkdocs<2` so we don't need to do
anything further here.

Author: StevenMaude

justfile

@@ -125,11 +125,11 @@ fix: devenv
 
 # Run the dev project
 run: devenv
-    $BIN/mkdocs serve -a localhost:8910
+    NO_MKDOCS_2_WARNING=1 $BIN/mkdocs serve -a localhost:8910
 
 # Build the documentation
 build: devenv
-    $BIN/mkdocs build --strict
+    NO_MKDOCS_2_WARNING=1 $BIN/mkdocs build --strict
 
 # Count words in generated documentation content (within <article> tags)
 wordcount: build