fix: rollback engines to >=16 (#383)

This is a partial revert of PR #374

It was dropping support for node@16 in CI but it didn't modify target in
tsconfig so 16 is still supported.

Furthermore, 16 is going to be supported by Vercel a little longer.

https://vercel.com/changelog/node-js-14-and-16-are-being-deprecated

This fixes the warning when installing Vercel CLI using node@16

```
npm WARN EBADENGINE Unsupported engine {
npm WARN EBADENGINE   package: '@vercel/nft@0.26.1',
npm WARN EBADENGINE   required: { node: '>=18' },
npm WARN EBADENGINE   current: { node: 'v16.18.1', npm: '8.19.2' }
npm WARN EBADENGINE }
```

package.json

@@ -132,7 +132,7 @@
     "when": "^3.7.8"
   },
   "engines": {
-    "node": ">=18"
+    "node": ">=16"
   },
   "packageManager": "npm@10.2.5",
   "publishConfig": {