GitHub Actions: Add Node.js 18 and drop 12

Node.js 12 is end-of-life. https://nodejs.org/en/about/releases

Python 3.6 is end-of-life.  https://devguide.python.org/#status-of-python-branches

Author: cclauss

.github/workflows/tests.yml

@@ -8,13 +8,13 @@ jobs:
       fail-fast: false
       max-parallel: 15
       matrix:
-        node: [12.x, 14.x, 16.x]
-        python: ["3.6", "3.8", "3.10"]
+        node: [14.x, 16.x, 18.x]
+        python: ["3.7", "3.8", "3.10"]
         os: [macos-latest, ubuntu-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout Repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Use Node.js ${{ matrix.node }}
         uses: actions/setup-node@v3
         with: