ci(npm): use Node 15 for running tests on CI

The `package-lock.json` is now created with NPM 7 (`lockfileVersion@2). The `setup-node@v2` uses NPM 7 only if `node-version` is set to be `15` (source: actions/setup-node#213)

Author: masakudamatsu

.github/workflows/validate.yml

@@ -9,7 +9,7 @@ jobs:
       - name: 'Install Node'
         uses: actions/setup-node@v2
         with:
-          node-version: '14'
+          node-version: '15'
           cache: 'npm'
       - name: 'Install repo'
         run: npm install