Drop Node.js 8.x from workflow.yml

```
/home/runner/work/node-lambda/node-lambda/node_modules/fs-extra/lib/mkdirs/make-dir.js:86
      } catch {
              ^

SyntaxError: Unexpected token {
```

.github/workflows/workflow.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        node-version: [8.x, 10.x, 12.x]
+        node-version: [10.x, 12.x]
         os: [ubuntu-latest, windows-latest, macos-latest]
     steps:
     - uses: actions/checkout@master