fix(deps): update dependency @google-cloud/iot to v2 (#326)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [@google-cloud/iot](https://github.com/googleapis/nodejs-iot) | dependencies | major | [`^1.0.0` -> `^2.0.0`](https://renovatebot.com/diffs/npm/@google-cloud%2fiot/1.8.0/2.0.0) |

---

### Release Notes

<details>
<summary>googleapis/nodejs-iot</summary>

### [`v2.0.0`](https://github.com/googleapis/nodejs-iot/blob/master/CHANGELOG.md#&#8203;200-httpswwwgithubcomgoogleapisnodejs-iotcomparev180v200-2020-04-12)

[Compare Source](https://github.com/googleapis/nodejs-iot/compare/v1.8.0...v2.0.0)

##### ⚠ BREAKING CHANGES

-   The library now supports Node.js v10+. The last version to support Node.js v8 is tagged legacy-8 on NPM.

##### Features

-   drop node8 support, support for async iterators ([#&#8203;297](https://github.com/googleapis/nodejs-iot/issues/297)) ([1cf607e](https://github.com/googleapis/nodejs-iot/commit/1cf607edcecfe2053596096efaa1cabe0d11de1e))

##### Bug Fixes

-   export explicit version from protos.js ([#&#8203;302](https://github.com/googleapis/nodejs-iot/issues/302)) ([8c8acba](https://github.com/googleapis/nodejs-iot/commit/8c8acba58f2670139863849cb86e3d8fe9fc7af7))
-   remove eslint, update gax, fix generated protos, run the generator ([#&#8203;306](https://github.com/googleapis/nodejs-iot/issues/306)) ([8edd1b5](https://github.com/googleapis/nodejs-iot/commit/8edd1b51347efc7a78390702ba8bd0a714599695))

</details>

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samples/manager/package.json

@@ -17,7 +17,7 @@
     "system-test": "mocha system-test/*.test.js --timeout=600000"
   },
   "dependencies": {
-    "@google-cloud/iot": "^1.6.1",
+    "@google-cloud/iot": "^2.0.0",
     "@google-cloud/pubsub": "^1.2.0",
     "googleapis": "^49.0.0",
     "mocha": "^7.1.1",

samples/mqtt_example/package.json

@@ -17,7 +17,7 @@
     "system-test": "mocha system-test/*.test.js --timeout=600000"
   },
   "dependencies": {
-    "@google-cloud/iot": "^1.0.0",
+    "@google-cloud/iot": "^2.0.0",
     "@google-cloud/pubsub": "^1.0.0",
     "jsonwebtoken": "^8.5.0",
     "mocha": "^7.0.0",