add node 18, remove 12 in main

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        node: [12, 14, 16]
+        node: [14, 16, 18]
     name: Test on Node v${{ matrix.node }}
     steps:
       - name: Checkout