ci: drop node 18 from matrix, vitest 4 requires node >=20

Author: gr2m

.github/workflows/test.yml

@@ -12,7 +12,6 @@ jobs:
     strategy:
       matrix:
         node_version:
-          - 18
           - 20
           - 22
 

package.json

@@ -73,6 +73,6 @@
     ]
   },
   "engines": {
-    "node": ">=18"
+    "node": ">=20"
   }
 }