fix(deps): update dependency gaxios to v4 (#420)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [gaxios](https://togithub.com/googleapis/gaxios) | dependencies | major | [`^3.0.0` -> `^4.0.0`](https://renovatebot.com/diffs/npm/gaxios/3.2.0/4.0.0) |

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### Release Notes

<details>
<summary>googleapis/gaxios</summary>

### [`v4.0.0`](https://togithub.com/googleapis/gaxios/blob/master/CHANGELOG.md#&#8203;400-httpswwwgithubcomgoogleapisgaxioscomparev320v400-2020-10-21)

[Compare Source](https://togithub.com/googleapis/gaxios/compare/v3.2.0...v4.0.0)

##### ⚠ BREAKING CHANGES

-   parameters in `url` and parameters provided via params will now be combined.

##### Bug Fixes

-   drop requirement on URL/combine url and params ([#&#8203;338](https://www.github.com/googleapis/gaxios/issues/338)) ([e166bc6](https://www.github.com/googleapis/gaxios/commit/e166bc6721fd979070ab3d9c69b71ffe9ee061c7))

</details>

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package.json

@@ -39,7 +39,7 @@
   "author": "Stephen Sawchuk",
   "license": "Apache-2.0",
   "dependencies": {
-    "gaxios": "^3.0.0",
+    "gaxios": "^4.0.0",
     "json-bigint": "^1.0.0"
   },
   "devDependencies": {