Update dependency gtoken to v5 (#925)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [gtoken](https://togithub.com/google/node-gtoken) | dependencies | major | [`^4.1.0` -> `^5.0.0`](https://renovatebot.com/diffs/npm/gtoken/4.1.4/5.0.0) |

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### Release Notes

<details>
<summary>google/node-gtoken</summary>

### [`v5.0.0`](https://togithub.com/google/node-gtoken/blob/master/CHANGELOG.md#&#8203;500httpswwwgithubcomgoogleapisnode-gtokencomparev414v500-2020-03-24)

[Compare Source](https://togithub.com/google/node-gtoken/compare/v4.1.4...v5.0.0)

##### ⚠ BREAKING CHANGES

-   drop Node 8 from engines ([#&#8203;284](https://togithub.com/google/node-gtoken/issues/284))
-   typescript@3.7.x introduced breaking changes to compiled code

##### Features

-   drop Node 8 from engines ([#&#8203;284](https://www.github.com/googleapis/node-gtoken/issues/284)) ([209e007](https://www.github.com/googleapis/node-gtoken/commit/209e00746116a82a3cf9acc158aff12a4971f3d0))

##### Build System

-   update gts and typescript ([#&#8203;283](https://www.github.com/googleapis/node-gtoken/issues/283)) ([ff076dc](https://www.github.com/googleapis/node-gtoken/commit/ff076dcb3da229238e7bed28d739c48986652c78))

##### [4.1.4](https://www.github.com/googleapis/node-gtoken/compare/v4.1.3...v4.1.4) (2020-01-06)

##### Bug Fixes

-   **deps:** pin TypeScript below 3.7.0 ([f1ae7b6](https://www.github.com/googleapis/node-gtoken/commit/f1ae7b64ead1c918546ae5bbe8546dfb4ecc788a))
-   **deps:** update dependency jws to v4 ([#&#8203;251](https://www.github.com/googleapis/node-gtoken/issues/251)) ([e13542f](https://www.github.com/googleapis/node-gtoken/commit/e13542f888a81ed3ced0023e9b78ed25264b1d1c))

##### [4.1.3](https://www.github.com/googleapis/node-gtoken/compare/v4.1.2...v4.1.3) (2019-11-15)

##### Bug Fixes

-   **deps:** use typescript ~3.6.0 ([#&#8203;246](https://www.github.com/googleapis/node-gtoken/issues/246)) ([5f725b7](https://www.github.com/googleapis/node-gtoken/commit/5f725b71f080e83058b1a23340acadc0c8704123))

##### [4.1.2](https://www.github.com/googleapis/node-gtoken/compare/v4.1.1...v4.1.2) (2019-11-13)

##### Bug Fixes

-   **docs:** add jsdoc-region-tag plugin ([#&#8203;242](https://www.github.com/googleapis/node-gtoken/issues/242)) ([994c5cc](https://www.github.com/googleapis/node-gtoken/commit/994c5ccf92731599aa63b84c29a9d5f6b1431cc5))

##### [4.1.1](https://www.github.com/googleapis/node-gtoken/compare/v4.1.0...v4.1.1) (2019-10-31)

##### Bug Fixes

-   **deps:** update gaxios to 2.1.0 ([#&#8203;238](https://www.github.com/googleapis/node-gtoken/issues/238)) ([bb12064](https://www.github.com/googleapis/node-gtoken/commit/bb1206420388399ef8992efe54c70bdb3fdcd965))

</details>

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package.json

@@ -23,7 +23,7 @@
     "fast-text-encoding": "^1.0.0",
     "gaxios": "^3.0.0",
     "gcp-metadata": "^4.0.0",
-    "gtoken": "^4.1.0",
+    "gtoken": "^5.0.0",
     "jws": "^4.0.0",
     "lru-cache": "^5.0.0"
   },