ci: don't bother testing on node 18

Author: isaacs

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     strategy:
       matrix:
-        node-version: [18.x, 22.x, 24.x]
+        node-version: [20.x, 22.x, 24.x]
         platform:
         - os: ubuntu-latest
           shell: bash