fix: pin @types/node to ^20 to fix build with TypeScript 4.x

Co-Authored-By: rlamb@launchdarkly.com <rlamb@launchdarkly.com>

Author: devin-ai-integration[bot]

package.json

@@ -36,6 +36,7 @@
     "@launchdarkly/node-server-sdk": "9.x",
     "@openfeature/server-sdk": "^1.16.0",
     "@types/jest": "^29.5.14",
+    "@types/node": "^20.19.35",
     "@typescript-eslint/eslint-plugin": "^5.22.0",
     "@typescript-eslint/parser": "^5.22.0",
     "eslint": "^8.14.0",