Bump version of nodejs

16 is also an LTS, and is the current active LTS.

Triggered by code-server package needing a newer version of nodejs!

repo2docker/buildpacks/conda/environment.yml

@@ -2,7 +2,7 @@ channels:
   - conda-forge
 dependencies:
   - python=3.7
-  - nodejs=14
+  - nodejs=16
   - pip
   - ipywidgets==7.7.0
   - jupyter-offlinenotebook==0.2.2