fix: require min node v18.16.1 for global fetch

v18 introduces global fetch now needed by oktokit.  v18.16.1 includes security
fixes.

Signed-off-by: Chris. Webster <[email protected]>

Author: webstech

package-lock.json

@@ -76,6 +76,6 @@
     "rfc2047": "^4.0.1"
   },
   "engines": {
-    "node": ">= 16.13.0"
+    "node": ">= 18.16.1"
   }
 }