Stop testing node 16.x

Ember CLI isn't supporting this yet, so let's not waste time testing it.

Author: jrjohnson

.github/workflows/ci.yaml

@@ -17,7 +17,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [12.x, 14.x, 16.x]
+        node-version: [12.x, 14.x]
 
     steps:
     - uses: actions/checkout@v2