fix: drop Node 18 from CI matrix (vitest 4.x requires Node >= 20)

Author: homeofe

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18, 20, 22]
+        node-version: [20, 22]
     steps:
       - uses: actions/checkout@v4
 

package.json

@@ -11,7 +11,7 @@
     }
   },
   "engines": {
-    "node": ">=18"
+    "node": ">=20"
   },
   "scripts": {
     "build": "tsc",