Update GH Actions CI setup

Author: rwjblue

.github/workflows/build.yml

@@ -26,11 +26,10 @@ jobs:
           - beta
           - canary
           - lts-3.16
-          - lts-3.12
-          - lts-3.8
-          - lts-3.4
-          - lts-2.18
-          - embroider
+          - lts-3.20
+          - embroider-safe
+          - embroider-optimized
+
     steps:
       - name: Checkout
         uses: actions/checkout@v1