fix(CI): update test-documentation to use Node 16.x, not 14.x

The documentation repo recently started requiring Node
16.x (Agoric/documentation#780), but the
agoric-sdk CI was still using 14.x . This changes the CI to use the
newer version.

.github/workflows/test-documentation.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: ['14.x']
+        node-version: ['16.x']
 
     steps:
       - name: Checkout agoric-sdk