chore(deps): update dependency @types/node to v18 (#1728)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [@types/node](https://togithub.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/node) ([source](https://togithub.com/DefinitelyTyped/DefinitelyTyped)) | [`^16.0.0` -> `^18.0.0`](https://renovatebot.com/diffs/npm/@types%2fnode/16.18.2/18.11.7) | [![age](https://badges.renovateapi.com/packages/npm/@types%2fnode/18.11.7/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/npm/@types%2fnode/18.11.7/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/npm/@types%2fnode/18.11.7/compatibility-slim/16.18.2)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/npm/@types%2fnode/18.11.7/confidence-slim/16.18.2)](https://docs.renovatebot.com/merge-confidence/) |

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system-test/fixtures/sample/package.json

@@ -16,7 +16,7 @@
     "@google-cloud/spanner": "file:./spanner.tgz"
   },
   "devDependencies": {
-    "@types/node": "^16.0.0",
+    "@types/node": "^18.0.0",
     "typescript": "^3.0.0",
     "gts": "^3.0.0"
   }