update CI workflow to use only node version 20

Node version 20 is the version listed in the tool-versions, and several
dependencies require 20 as the minimum version.

.github/workflows/ci-workflow.yml

@@ -10,7 +10,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v3
         with:
-          node-version: '18.x'
+          node-version: '20.x'
       - run: npm install
       - run: npm run lint
       - run: npm run prettier
@@ -22,7 +22,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        node-version: [18, 20]
+        node-version: [20]
 
     steps:
       - uses: actions/checkout@v4