tools: use Node.js 16.x for GitHub workflow

find-inactive-collaborators.mjs works fine with Node.js 16.x, but GitHub
Actions currently use 14.x by default.

PR-URL: #39362
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

.github/workflows/find-inactive-collaborators.yml

@@ -13,5 +13,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
-    - run: tools/find-inactive-collaborators.mjs '1 year ago'
+      - uses: actions/checkout@v2
+
+      - name: Install Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: 16.x
+
+      - name: Find inactive collaborators
+        run: tools/find-inactive-collaborators.mjs '1 year ago'