ci: replace Node.js 15 with 16

Author: iiroj

.github/workflows/main.yml

@@ -20,12 +20,12 @@ jobs:
   test:
     strategy:
       matrix:
-        # Test with Node.js v10 (LTS), v12 (LTS), v14 (LTS), and v15 (latest)
+        # Test with Node.js v10 (LTS), v12 (LTS), v14 (LTS), and v16 (latest)
         node:
           - 10
           - 12
           - 14
-          - 15
+          - 16
         # Test with Ubuntu and macOS
         os:
           - ubuntu-latest