chore: bump minimum Node.js version to 18.18.0

- Updated from Node.js 12+ to 18.18+ to match ESLint 9 requirements
- Node.js 12, 14, and 16 are all end-of-life
- Aligns with GitHub Actions test matrix (18.x, 20.x, 22.x, 24.x)

Author: andris9

package.json

@@ -40,6 +40,6 @@
         "SMTP"
     ],
     "engines": {
-        "node": ">=12.0.0"
+        "node": ">=18.18.0"
     }
 }