ci: change node 8 -> node 13

Author: nodkz

.github/workflows/nodejs.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [8.x, 10.x, 12.x]
+        node-version: [10.x, 12.x, 13.x]
     steps:
     - uses: actions/checkout@v1
     - name: Use Node.js ${{ matrix.node-version }}