ci: update 'node-version'

https://github.com/nodejs/Release

- Drop 15.x
- Add 17.x

Author: abetomo

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        node-version: [12.x, 14.x, 15.x, 16.x]
+        node-version: [12.x, 14.x, 16.x, 17.x]
         os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps: