change node version to minimum (18)

Author: nicknisi

.github/workflows/runtime-tests.yml

@@ -9,7 +9,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version: '18'
       - uses: denoland/setup-deno@v2
         with:
           deno-version: v1.x
@@ -22,4 +22,3 @@ jobs:
 
       - name: Runtime compatibility check
         run: npm run check:runtimes
-