build: stop running Node 12 & 14 tests

Our dependencies have now moved past Node 12 & 14, and several of them
no longer support these versions of Node.

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        node: [12, 14, 16]
+        node: [16]
     steps:
     - name: Checkout
       uses: actions/checkout@v2