Downgrade nodejs and mystmd.

I'm checking whether a navigation problem on GitHub Pages was caused by a nodejs or mystmd update. This reverse nodejs to >=18.0 has before and mystmd to 1.1.56. We had been previously on 1.1.55.

environment.yml

@@ -5,6 +5,6 @@ channels:
 
 dependencies:
   - python==3.11.*
-  - nodejs>=20.0
+  - nodejs>=18.0
   - libwebp==1.3.2
-  - mystmd==1.2.0
+  - mystmd==1.1.56